The lava lake in the summit crater of Erebus has been active since at least 1972. Located in Antarctica overlooking the McMurdo Station on Ross Island, it is the southernmost active volcano on the planet. Because of the remote location, activity is primarily monitored by satellites. This report covers activity during 2023.

The number of thermal alerts recorded by the Hawai'i Institute of Geophysics and Planetology’s MODVOLC Thermal Alerts System increased considerably in 2023 compared to the years 2020-2022 (table 9). In contrast to previous years, the MODIS instruments aboard the Aqua and Terra satellites captured data from Erebus every month during 2023. Consistent with previous years, the lowest number of anomalous pixels were recorded in January, November, and December.

Table 9. Number of monthly MODIS-MODVOLC thermal alert pixels recorded at Erebus during 2017-2023. See BGVN 42:06 for data from 2000 through 2016. The table was compiled using data provided by the HIGP – MODVOLC Thermal Alerts System.

Sentinel-2 infrared images showed one or two prominent heat sources within the summit crater, accompanied by adjacent smaller sources, similar to recent years (see BGVN 46:01, 47:02, and 48:01). A unique image was obtained on 25 November 2023 by the OLI-2 (Operational Land Imager-2) on Landsat 9, showing the upper part of the volcano surrounded by clouds (figure 32).

Figure 32. Satellite view of Erebus with the summit and upper flanks visible above the surrounding weather clouds on 25 November 2023. Landsat 9 OLI-2 (Operational Land Imager-2) image with visible and infrared bands. Thermal anomalies are present in the summit crater. The edifice is visible from about 2,000 m elevation to the summit around 3,800 m. The summit crater is ~500 m in diameter, surrounded by a zone of darker snow-free deposits; the larger circular summit area is ~4.5 km diameter. NASA Earth Observatory image by Lauren Dauphin, using Landsat data from the U.S. Geological Survey.

Geologic Background. Mount Erebus, the world's southernmost historically active volcano, overlooks the McMurdo research station on Ross Island. It is the largest of three major volcanoes forming the crudely triangular Ross Island. The summit of the dominantly phonolitic volcano has been modified by one or two generations of caldera formation. A summit plateau at about 3,200 m elevation marks the rim of the youngest caldera, which formed during the late-Pleistocene and within which the modern cone was constructed. An elliptical 500 x 600 m wide, 110-m-deep crater truncates the summit and contains an active lava lake within a 250-m-wide, 100-m-deep inner crater; other lava lakes are sometimes present. The glacier-covered volcano was erupting when first sighted by Captain James Ross in 1841. Continuous lava-lake activity with minor explosions, punctuated by occasional larger Strombolian explosions that eject bombs onto the crater rim, has been documented since 1972, but has probably been occurring for much of the volcano's recent history.

Information Contacts: Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: https://earthobservatory.nasa.gov/images/152134/erebus-breaks-through).

Rincón de la Vieja is a volcanic complex in Costa Rica with a hot convecting acid lake that exhibits frequent weak phreatic explosions, gas-and-steam emissions, and occasional elevated sulfur dioxide levels (BGVN 45:10, 46:03, 46:11). The current eruption period began June 2021. This report covers activity during July-December 2023 and is based on weekly bulletins and occasional daily reports from the Observatorio Vulcanologico Sismologica de Costa Rica-Universidad Nacional (OVSICORI-UNA).

Numerous weak phreatic explosions continued during July-December 2023, along with gas-and-steam emissions and plumes that rose as high as 3 km above the crater rim. Many weekly OVSICORI-UNA bulletins included the previous week's number of explosions and emissions (table 9). For many explosions, the time of explosion was given (table 10). Frequent seismic activity (long-period earthquakes, volcano-tectonic earthquakes, and tremor) accompanied the phreatic activity.

Table 9. Number of reported weekly phreatic explosions and gas-and-steam emissions at Rincón de la Vieja, July-December 2023. Counts are reported for the week before the Weekly Bulletin date; not all reports included these data. Courtesy of OVSICORI-UNA.

Table 10. Summary of activity at Rincón de la Vieja during July-December 2023. Weak phreatic explosions and gas emissions are noted where the time of explosion was indicated in the weekly or daily bulletins. Height of plumes or emissions are distance above the crater rim. Courtesy of OVSICORI-UNA.

According to OVSICORI-UNA, during July-October the average weekly sulfur dioxide (SO₂) flux ranged from 68 to 240 tonnes/day. However, in mid-November the flux increased to as high as 334 tonnes/day, the highest value measured in recent years. The high SO₂ flux in mid-November was also detected by the TROPOMI instrument on the Sentinel-5P satellite (figure 43).

Figure 43. Sulfur dioxide (SO2) maps from Rincón de la Vieja recorded by the TROPOMI instrument aboard the Sentinel-5P satellite on 16 November (left) and 20 November (right) 2023. Mass estimates are consistent with measurements by OVSICORI-UNA near ground level. Some of the plume on 20 November may be from other volcanoes (triangle symbols) in Costa Rica and Nicaragua. Courtesy of the NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/).

Bezymianny, located on Russia’s Kamchatka Peninsula, has had eruptions since 1955 characterized by dome growth, explosions, pyroclastic flows, ash plumes, and ashfall. Activity during November 2022-April 2023 included gas-and-steam emissions, lava dome collapses generating avalanches, and persistent thermal activity. Similar eruptive activity continued from May through October 2023, described here based on information from weekly and daily reports of the Kamchatka Volcano Eruptions Response Team (KVERT), notices from Tokyo VAAC (Volcanic Ash Advisory Center), and from satellite data.

Overall activity decreased after the strong period of activity in late March through April 2023, which included ash explosions during 29 March and 7-8 April 2023 that sent plumes as high as 10-12 km altitude, along with dome growth and lava flows (BGVN 48:05). This reduced activity can be seen in the MIROVA thermal detection system graph (figure 56), which was consistent with data from the MODVOLC thermal detection system and with Sentinel-2 satellite images that showed persistent hotspots in the summit crater when conditions allowed observations. A renewed period of strong activity began in mid-October 2023.

Figure 56. The MIROVA (Log Radiative Power) thermal data for Bezymianny during 20 November 2022 through October 2023 shows heightened activity in the first half of April and second half of October 2023, with lower levels of thermal anomalies in between those times. Courtesy of MIROVA.

Activity increased significantly on 17 October 2023 when large collapses began during 0700-0830 on the E flanks of the lava dome and continued to after 0930 the next day (figure 57). Ash plumes rose to an altitude of 4.5-5 km, extending 220 km NNE by 18 October. A large explosion at 1630 on 18 October produced an ash plume that rose to an altitude of 11 km (8 km above the summit) and drifted NNE and then NW, extending 900 km NW within two days at an altitude of 8 km. Minor ashfall was noted in Kozyrevsk (45 km WNW). At 0820 on 20 October an ash plume was identified in satellite images drifting 100 km ENE at altitudes of 4-4.5 km.

Figure 57. Sentinel-2 satellite images of Bezymianny from 1159 on 17 October 2023 (2359 on 16 October UTC) showing a snow-free S and SE flank along with thermal anomalies in the crater and down the SE flank. Left image is in false color (bands 8, 4, 3); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Lava flows and hot avalanches from the dome down the SE flank continued over the next few days, including 23 October when clear conditions allowed good observations (figures 58 and 59). A large thermal anomaly was observed over the volcano through 24 October, and in the summit crater on 30 October (figure 60). Strong fumarolic activity continued, with numerous avalanches and occasional incandescence. By the last week of October, volcanic activity had decreased to a level consistent with that earlier in the reporting period.

Figure 58. Daytime photo of Bezymianny under clear conditions on 23 October 2023 showing a lava flow and avalanches descending the SE flank, incandescence from the summit crater, and a small ash plume. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 59. Night photo of Bezymianny under cloudy conditions on 23 October 2023 showing an incandescent lava flow and avalanches descending the SE flank. Photo by Yu. Demyanchuk, courtesy of IVS FEB RAS, KVERT.

Figure 60. Sentinel-2 satellite images of Bezymianny from 1159 on 30 October 2023 (2359 on 29 October UTC) showing a plume drifting SE and thermal anomalies in the summit crater and down multiple flanks. Left image is in true color (bands 4, 3, 2); right image is thermal infrared (bands 12, 11, 8A). Courtesy of Copernicus Browser.

Aviation warnings were frequently updated during 17-20 October. KVERT issued a Volcano Observatory Notice for Aviation (VONA) on 17 October at 1419 and 1727 (0219 and 0527 UTC) raising the Aviation Color Code (ACC) from Yellow to Orange (second highest level). The next day, KVERT issued a VONA at 1705 (0505 UTC) raising the ACC to Red (highest level) but lowered it back to Orange at 2117 (0917 UTC). After another decrease to Yellow and back to Orange, the ACC was reduced to Yellow on 20 October at 1204 (0004 UTC). In addition, the Tokyo VAAC issued a series of Volcanic Ash Advisories beginning on 16 October and continuing through 30 October.

Geologic Background. The modern Bezymianny, much smaller than its massive neighbors Kamen and Kliuchevskoi on the Kamchatka Peninsula, was formed about 4,700 years ago over a late-Pleistocene lava-dome complex and an edifice built about 11,000-7,000 years ago. Three periods of intensified activity have occurred during the past 3,000 years. The latest period, which was preceded by a 1,000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large open crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Kamchatka Volcanological Station, Kamchatka Branch of Geophysical Survey, (KB GS RAS), Klyuchi, Kamchatka Krai, Russia (URL: http://volkstat.ru/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).chr

Kīlauea is the southeastern-most volcano in Hawaii and overlaps the E flank of the Mauna Loa volcano. Its East Rift Zone (ERZ) has been intermittently active for at least 2,000 years. An extended eruption period began in January 1983 and was characterized by open lava lakes and lava flows from the summit caldera and the East Rift Zone. During May 2018 magma migrated into the Lower East Rift Zone (LERZ) and opened 24 fissures along a 6-km-long NE-trending fracture zone that produced lava flows traveling in multiple directions. As lava emerged from the fissures, the lava lake at Halema'uma'u drained and explosions sent ash plumes to several kilometers altitude (BGVN 43:10).

The current eruption period started during September 2021 and has recently been characterized by lava effusions, spatter, and sulfur dioxide emissions in the active Halema’uma’u lava lake (BGVN 47:08). Lava effusions, some spatter, and sulfur dioxide emissions have continued during this reporting period of July through December 2022 using daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Summary of activity during July-December 2022. Low-level effusions have continued at the western vent of the Halema’uma’u crater during July through early December 2022. Occasional weak ooze-outs (also called lava break outs) would occur along the margins of the crater floor. The overall level of the active lava lake throughout the reporting period gradually increased due to infilling, however it stagnated in mid-September (table 13). During September through November, activity began to decline, though lava effusions persisted at the western vent. By 9 December, the active part of the lava lake had completely crusted over, and incandescence was no longer visible.

Table 13. Summary of measurements taken during overflights at Kīlauea that show a gradual increase in the active lava lake level and the volume of lava effused since 29 September 2021. Lower activity was reported during September-October. Data collected during July-December 2022. Courtesy of HVO.

Activity during July 2022. Lava effusions were reported from the western vent in the Halema’uma’u crater, along with occasional weak ooze-outs along the margins of the crater floor. The height of the lava lake was variable due to deflation-inflation tilt events; for example, the lake level dropped approximately 3-4 m during a summit deflation-inflation event reported on 1 July. Webcam images taken during the night of 6-12 July showed intermittent low-level spattering at the western vent that rose less than 10 m above the vent (figure 519). Measurements made during an overflight on 7 July indicated that the crater floor was infilled about 130 m and that 95 million cubic meters of lava had been effused since 29 September 2021. A single, relatively small lava ooze-out was active to the S of the lava lake. Around midnight on 8 July there were two brief periods of lava overflow onto the lake margins. On 9 July lava ooze-outs were reported near the SE and NE edges of the crater floor and during 10-11 July they occurred near the E, NE, and NW edges. On 16 July crater incandescence was reported, though the ooze-outs and spattering were not visible. On 18 July overnight webcam images showed incandescence in the western vent complex and two ooze-outs were reported around 0000 and 0200 on 19 July. By 0900 there were active ooze-outs along the SW edge of the crater floor. Measurements made from an overflight on 19 July indicated that the crater floor was infilled about 133 m and 98 million cubic meters of lava had erupted since 29 September 2021 (figure 520). On 20 July around 1600 active ooze-outs were visible along the N edge of the crater, which continued through the next day. Extensive ooze-outs occurred along the W margin during 24 July until 1900; on 26 July minor ooze-outs were noted along the N margin. Minor spattering was visible on 29 July along the E margin of the lake. The sulfur dioxide emission rates ranged 650-2,800 tons per day (t/d), the higher of which was measured on 8 July (figure 519).

Figure 519. Minor spattering rising less than 10 m was visible at the E end of the lava lake within Halema‘uma‘u, at the summit of Kīlauea on 8 July 2022. Sulfur dioxide is visible rising from the lake surface (bluish-colored fume). A sulfur dioxide emission rate of approximately 2,800 t/d was measured on 8 July. Courtesy of K. Mulliken, USGS.

Figure 520. A helicopter overflight on 19 July 2022 allowed for aerial visible and thermal imagery to be taken of the Halema’uma’u crater at Kīlauea’s summit crater. The active part of the lava lake is confined to the western part of the crater. The scale of the thermal map ranges from blue to red, with blue colors indicative of cooler temperatures and red colors indicative of warmer temperatures. Courtesy of USGS, HVO.

Activity during August 2022. The eruption continued in the Halema’uma’u crater at the western vent. According to HVO the lava in the active lake remained at the level of the bounding levees. Occasional minor ooze-outs were observed along the margins of the crater floor. Strong nighttime crater incandescence was visible after midnight on 6 August over the western vent cone. During 6-7 August scattered small lava lobes were active along the crater floor and incandescence persisted above the western vent through 9 August. During 7-9 August HVO reported a single lava effusion source was active along the NW margin of the crater floor. Measurements from an overflight on 4 August indicated that the crater floor was infilled about 136 m total and that 102 million cubic meters of lava had been erupted since the start of the eruption. Lava breakouts were reported along the N, NE, E, S, and W margins of the crater during 10-16 August. Another overflight survey conducted on 16 August indicated that the crater floor infilled about 137 m and 104 million cubic meters of lava had been erupted since September 2021. Measured sulfur dioxide emissions rates ranged 1,150-2,450 t/d, the higher of which occurred on 8 August.

Activity during September 2022. During September, lava effusion continued from the western vent into the active lava lake and onto the crater floor. Intermittent minor ooze-outs were reported through the month. A small ooze-out was visible on the W crater floor margin at 0220 on 2 September, which showed decreasing surface activity throughout the day, but remained active through 3 September. On 3 September around 1900 a lava outbreak occurred along the NW margin of the crater floor but had stopped by the evening of 4 September. Field crews monitoring the summit lava lake on 9 September observed spattering on the NE margin of the lake that rose no higher than 10 m, before falling back onto the lava lake crust (figure 521). Overflight measurements on 12 September indicated that the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had been erupted since September 2021. Extensive breakouts in the W and N part of the crater floor were reported at 1600 on 20 September and continued into 26 September. The active part of the lava lake dropped by 10 m while other parts of the crater floor dropped by several meters. Summit tiltmeters recorded a summit seismic swarm of more than 80 earthquakes during 1500-1800 on 21 September, which occurred about 1.5 km below Halema’uma’u; a majority of these were less than Mw 2. By 22 September the active part of the lava lake was infilled about 2 m. On 23 September the western vent areas exhibited several small spatter cones with incandescent openings, along with weak, sporadic spattering (figure 522). The sulfur dioxide emission rate ranged from 930 t/d to 2,000 t/d, the higher of which was measured on 6 September.

Figure 521. Photo of spattering occurring at Kīlauea's Halema’uma’u crater during the morning of 9 September 2022 on the NE margin of the active lava lake. The spatter material rose 10 m into the air before being deposited back on the lava lake crust. Courtesy of C. Parcheta, USGS.

Figure 522.The active western vent area at Kīlauea's Halema’uma’u crater consisted of several small spatter cones with incandescent openings and weak, sporadic spattering. Courtesy of M. Patrick, USGS.

Activity during October 2022. Activity during October declined slightly compared to previous months, though lava effusions persisted from the western vent into the active lava lake and onto the crater floor during October (figure 523). Slight variations in the lava lake were noted throughout the month. HVO reported that around 0600 on 3 October the level of the lava lake has lowered slightly. Overflight measurements taken on 5 October indicated that the crater floor was infilled a total of about 143 m and that 111 million cubic meters of lava had been effused since September 2021. During 6-7 October the lake gradually rose 0.5 m. Sulfur dioxide measurements made on 22 October had an emission rate of 700 t/d. Another overflight taken on 28 October showed that there was little to no change in the elevation of the crater floor: the crater floor was infilled a total of 143 m and 111 million cubic meters of lava had erupted since the start of the eruption.

Figure 523. Photo of the Halema’uma’u crater at Kīlauea looking east from the crater rim showing the active lava lake, with active lava ponds to the SE (top) and west (bottom middle) taken on 5 October 2022. The western vent complex is visible through the gas at the bottom center of the photo. Courtesy of N. Deligne, USGS.

Activity during November 2022. Activity remained low during November, though HVO reported that lava from the western vent continued to effuse into the active lava lake and onto the crater floor throughout the month. The rate of sulfur dioxide emissions during November ranged from 300-600 t/d, the higher amount of which occurred on 9 November.

Activity during December 2022. Similar low activity was reported during December, with lava effusing from the western vent into the active lava lake and onto the crater floor. During 4-5 December the active part of the lava lake was slightly variable in elevation and fluctuated within 1 m. On 9 December HVO reported that lava was no longer erupting from the western vent in the Halema’uma’u crater and that sulfur dioxide emissions had returned to near pre-eruption background levels; during 10-11 December, the lava lake had completely crusted over, and no incandescence was visible (figure 524). Time lapse camera images covering the 4-10 December showed that the crater floor showed weak deflation and no inflation. Some passive events of crustal overturning were reported during 14-15 December, which brought fresh incandescent lava to the lake surface. The sulfur dioxide emission rate was approximately 200 t/d on 14 December. A smaller overturn event on 17 December and another that occurred around 0000 and into the morning of 20 December were also detected. A small seismic swarm was later detected on 30 December.

Figure 524. Photo of Halema’uma’u crater at Kīlauea showing a mostly solidified lake surface during the early morning of 10 December 2022. Courtesy of J. Bard, USGS.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: http://hvo.wr.usgs.gov/).

Nyamulagira (also known as Nyamuragira) is a shield volcano in the Democratic Republic of Congo with the summit truncated by a small 2 x 2.3 km caldera with walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from numerous flank fissures and cinder cones. The current eruption period began in April 2018 and has more recently been characterized by summit crater lava flows and thermal activity (BGVN 48:05). This report describes lava flows and variable thermal activity during May through October 2023, based on information from the Observatoire Volcanologique de Goma (OVG) and various satellite data.

Lava lake activity continued during May. The MIROVA (Middle InfraRed Observation of Volcanic Activity) system recorded moderate-to-strong thermal activity throughout the reporting period; activity was more intense during May and October and relatively weaker from June through September (figure 95). The MODVOLC thermal algorithm, detected a total of 209 thermal alerts. There were 143 hotspots detected during May, eight during June, nine during September, and 49 during October. This activity was also reflected in infrared satellite images, where a lava flow was visible in the NW part of the crater on 7 May and strong activity was seen in the center of the crater on 4 October (figure 96). Another infrared satellite image taken on 12 May showed still active lava flows along the NW margin of the crater. According to OVG lava effusions were active during 7-29 May and moved to the N and NW parts of the crater beginning on 9 May. Strong summit crater incandescence was visible from Goma (27 km S) during the nights of 17, 19, and 20 May (figure 97). On 17 May there was an increase in eruptive activity, which peaked at 0100 on 20 May. Notable sulfur dioxide plumes drifted NW and W during 19-20 May (figure 98). Drone footage acquired in partnership with the USGS (United States Geological Survey) on 20 May captured images of narrow lava flows that traveled about 100 m down the W flank (figure 99). Data from the Rumangabo seismic station indicated a decreasing trend in activity during 17-21 May. Although weather clouds prevented clear views of the summit, a strong thermal signature on the NW flank was visible in an infrared satellite image on 22 May, based on an infrared satellite image. On 28 May the lava flows on the upper W flank began to cool and solidify. By 29 May seismicity returned to levels similar to those recorded before the 17 May increase. Lava effusion continued but was confined to the summit crater; periodic crater incandescence was observed.

Figure 95. Moderate-to-strong thermal anomalies were detected at Nyamulagira during May through October 2023, as shown on this MIROVA graph (Log Radiative Power). During late May, the intensity of the anomalies gradually decreased and remained at relatively lower levels during mid-June through mid-September. During mid-September, the power of the anomalies gradually increased again. The stronger activity is reflective of active lava effusions. Courtesy of MIROVA.

Figure 96. Infrared (bands B12, B11, B4) satellite images showing a constant thermal anomaly of variable intensities in the summit crater of Nyamulagira on 7 May 2023 (top left), 21 June 2023 (top right), 21 July 2023 (bottom left), and 4 October 2023 (bottom right). Although much of the crater was obscured by weather clouds on 7 May, a possible lava flow was visible in the NW part of the crater. Courtesy of Copernicus Browser.

Figure 97. Photo of intense nighttime crater incandescence at Nyamulagira as seen from Goma (27 km S) on the evening of 19 May 2023. Courtesy of Charles Balagizi, OVG.

Figure 98. Two strong sulfur dioxide plumes were detected at Nyamulagira and drifted W on 19 (left) and 20 (right) May 2023. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Figure 99. A map (top) showing the active vents (yellow pins) and direction of active lava flows (W) at Nyamulagira at Virunga National Park on 20 May 2023. Drone footage (bottom) also shows the fresh lava flows traveling downslope to the W on 20 May 2023. Courtesy of USGS via OVG.

Low-level activity was noted during June through October. On 1 June OVG reported that seismicity remained at lower levels and that crater incandescence had been absent for three days, though infrared satellite imagery showed continued lava effusion in the summit crater. The lava flows on the flanks covered an estimated 0.6 km². Satellite imagery continued to show thermal activity confined to the lava lake through October (figure 96), although no lava flows or significant sulfur dioxide emissions were reported.

Geologic Background. Africa's most active volcano, Nyamulagira (also known as Nyamuragira), is a massive high-potassium basaltic shield about 25 km N of Lake Kivu and 13 km NNW of the steep-sided Nyiragongo volcano. The summit is truncated by a small 2 x 2.3 km caldera that has walls up to about 100 m high. Documented eruptions have occurred within the summit caldera, as well as from the numerous flank fissures and cinder cones. A lava lake in the summit crater, active since at least 1921, drained in 1938, at the time of a major flank eruption. Recent lava flows extend down the flanks more than 30 km from the summit as far as Lake Kivu; extensive lava flows from this volcano have covered 1,500 km2 of the western branch of the East African Rift.

Information Contacts: Observatoire Volcanologique de Goma (OVG), Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo; Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Charles Balagizi, Goma Volcano Observatory, Departement de Geophysique, Centre de Recherche en Sciences Naturelles, Lwiro, D.S. Bukavu, DR Congo.

The remote volcano of Bagana is located in central Bougainville Island, Papua New Guinea. Recorded eruptions date back to 1842 and activity has consisted of effusive activity that has built a small lava dome in the summit crater and occasional explosions that produced pyroclastic flows. The most recent eruption has been ongoing since February 2000 and has produced occasional explosions, ash plumes, and lava flows. More recently, activity has been characterized by ongoing effusive activity and ash emissions (BGVN 48:04). This report updates activity from April through September 2023 that has consisted of explosions, ash plumes, ashfall, and lava flows, using information from the Darwin Volcanic Ash Advisory Center (VAAC) and satellite data.

An explosive eruption was reported on 7 July that generated a large gas-and-ash plume to high altitudes and caused significant ashfall in local communities; the eruption plume had reached upper tropospheric (16-18 km altitude) altitudes by 2200, according to satellite images. Sulfur dioxide plumes were detected in satellite images on 8 July and indicated that the plume was likely a mixture of gas, ice, and ash. A report issued by the Autonomous Bougainville Government (ABG) (Torokina District, Education Section) on 10 July noted that significant ash began falling during 2000-2100 on 7 July and covered most areas in the Vuakovi, Gotana (9 km SW), Koromaketo, Laruma (25 km W) and Atsilima (27 km NW) villages. Pyroclastic flows also occurred, according to ground-based reports; small deposits confined to one drainage were inspected by RVO during an overflight on 17 July and were confirmed to be from the 7 July event. Ashfall continued until 10 July and covered vegetation, which destroyed bushes and gardens and contaminated rivers and streams.

RVO reported another eruption on 14 July. The Darwin VAAC stated that an explosive event started around 0830 on 15 July and produced an ash plume that rose to 16.5 km altitude by 1000 and drifted N, according to satellite images. The plume continued to drift N and remained visible through 1900, and by 2150 it had dissipated.

Ashfall likely from both the 7 and 15 July events impacted about 8,111 people in Torokina (20 km SW), including Tsito/Vuakovi, Gotana, Koromaketo, Kenaia, Longkogari, Kenbaki, Piva (13 km SW), and Atsinima, and in the Tsitovi district, according to ABG. Significant ashfall was also reported in Ruruvu (22 km N) in the Wakunai District of Central Bougainville, though the thickness of these deposits could not be confirmed. An evacuation was called for the villages in Wakunai, where heavy ashfall had contaminated water sources; the communities of Ruruvu, Togarau, Kakarapaia, Karauturi, Atao, and Kuritaturi were asked to evacuate to a disaster center at the Wakunai District Station, and communities in Torokina were asked to evacuate to the Piva District station. According to a news article, more than 7,000 people needed temporary accommodations, with about 1,000 people in evacuation shelters. Ashfall had deposited over a broad area, contaminating water supplies, affecting crops, and collapsing some roofs and houses in rural areas. Schools were temporarily shut down. Intermittent ash emissions continued through the end of July and drifted NNW, NW, and SW. Fine ashfall was reported on the coast of Torokina, and ash plumes also drifted toward Laruma and Atsilima.

A small explosive eruption occurred at 2130 on 28 July that ejected material from the crater vents, according to reports from Torokina, in addition to a lava flow that contained two lobes. A second explosion was detected at 2157. Incandescence from the lava flow was visible from Piva as it descended the W flank around 2000 on 29 July (figure 47). The Darwin VAAC reported that a strong thermal anomaly was visible in satellite images during 30-31 July and that ash emissions rose to 2.4 km altitude and drifted WSW on 30 July. A ground report from RVO described localized emissions at 0900 on 31 July.

Figure 47. Infrared (bands B12, B11, B4) satellite images showed weak thermal anomalies at the summit crater of Bagana on 12 April 2023 (top left), 27 May 2023 (top right), 31 July 2023 (bottom left), and 19 September 2023 (bottom right). A strong thermal anomaly was detected through weather clouds on 31 July and extended W from the summit crater. Courtesy of Copernicus Browser.

The Darwin VAAC reported that ash plumes were identified in satellite imagery at 0800 and 1220 on 12 August and rose to 2.1 km and 3 km altitude and drifted NW and W, respectively. A news report stated that aid was sent to more than 6,300 people that were adversely affected by the eruption. Photos taken during 17-19 August showed ash emissions rising no higher than 1 km above the summit and drifting SE. A small explosion generated an ash plume during the morning of 19 August. Deposits from small pyroclastic flows were also captured in the photos. Satellite images captured lava flows and pyroclastic flow deposits. Two temporary seismic stations were installed near Bagana on 17 August at distances of 7 km WSW (Vakovi station) and 11 km SW (Kepox station). The Kepox station immediately started to record continuous, low-frequency background seismicity.

Satellite data. Little to no thermal activity was detected during April through mid-July 2023; only one anomaly was recorded during early April and one during early June, according to MIROVA (Middle InfraRed Observation of Volcanic Activity) data (figure 48). Thermal activity increased in both power and frequency during mid-July through September, although there were still some short gaps in detected activity. MODVOLC also detected increased thermal activity during August; thermal hotspots were detected a total of five times on 19, 20, and 27 August. Weak thermal anomalies were also captured in infrared satellite images on clear weather days throughout the reporting period on 7, 12, and 17 April, 27 May, 1, 6, 16, and 31 July, and 19 September (figure 48); a strong thermal anomaly was visible on 31 July. Distinct sulfur dioxide plumes that drifted generally NW were intermittently captured by the TROPOMI instrument on the Sentinel-5P satellite and sometimes exceeded two Dobson Units (DUs) (figure 49).

Figure 48. Low thermal activity was detected at Bagana during April through mid-July 2023, as shown on this MIROVA graph. In mid-July, activity began to increase in both frequency and power, which continued through September. There were still some pauses in activity during late July, early August, and late September, but a cluster of thermal activity was detected during late August. Courtesy of MIROVA.

Figure 49. Distinct sulfur dioxide plumes rising from Bagana on 15 July 2023 (top left), 16 July 2023 (top right), 17 July 2023 (bottom left), and 17 August 2023 (bottom right). These plumes all generally drifted NW; a particularly notable plume exceeded 2 Dobson Units (DUs) on 15 July. Data is from the TROPOMI instrument on the Sentinel-5P satellite. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.0

Geologic Background. Bagana volcano, in a remote portion of central Bougainville Island, is frequently active. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although occasional explosive activity produces pyroclastic flows. Lava flows with tongue-shaped lobes up to 50 m thick and prominent levees descend the flanks on all sides.

Information Contacts: Rabaul Volcano Observatory (RVO), Geohazards Management Division, Department of Mineral Policy and Geohazards Management (DMPGM), PO Box 3386, Kokopo, East New Britain Province, Papua New Guinea; Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Autonomous Bougainville Government, P.O Box 322, Buka, AROB, PNG (URL: https://abg.gov.pg/); Andrew Tupper (Twitter: @andrewcraigtupp); Simon Carn, Geological and Mining Engineering and Sciences, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, USA (URL: http://www.volcarno.com/, Twitter: @simoncarn); Radio NZ (URL: https://www.rnz.co.nz/news/pacific/494464/more-than-7-000-people-in-bougainville-need-temporary-accommodation-after-eruption); USAID, 1300 Pennsylvania Ave, NW, Washington DC 20004, USA (URL: https://www.usaid.gov/pacific-islands/press-releases/aug-08-2023-united-states-provides-immediate-emergency-assistance-support-communities-affected-mount-bagana-volcanic-eruptions).

Mayon is located in the Philippines and has steep upper slopes capped by a small summit crater. Historical eruptions date back to 1616 CE that have been characterized by Strombolian eruptions, lava flows, pyroclastic flows, and mudflows. Eruptions mostly originated from a central conduit. Pyroclastic flows and mudflows have commonly descended many of the approximately 40 drainages that surround the volcano. The most recent eruption occurred during June through October 2022 and consisted of lava dome growth and gas-and-steam emissions (BGVN 47:12). A new eruption was reported during late April 2023 and has included lava flows, pyroclastic density currents, ash emissions, and seismicity. This report covers activity during April through September 2023 based on daily bulletins from the Philippine Institute of Volcanology and Seismology (PHIVOLCS).

During April through September 2023, PHIVOLCS reported near-daily rockfall events, frequent volcanic earthquakes, and sulfur dioxide measurements. Gas-and-steam emissions rose 100-900 m above the crater and drifted in different directions. Nighttime crater incandescence was often visible during clear weather and was accompanied by incandescent avalanches of material. Activity notably increased during June when lava flows were reported on the S, SE, and E flanks (figure 52). The MIROVA graph (Middle InfraRed Observation of Volcanic Activity) showed strong thermal activity coincident with these lava flows, which remained active through September (figure 53). According to the MODVOLC thermal algorithm, a total of 110 thermal alerts were detected during the reporting period: 17 during June, 40 during July, 27 during August, and 26 during September. During early June, pyroclastic density currents (PDCs) started to occur more frequently.

Figure 52. Infrared (bands B12, B11, B4) satellite images show strong lava flows descending the S, SE, and E flanks of Mayon on 13 June 2023 (top left), 23 June 2023 (top right), 8 July 2023 (bottom left), and 7 August 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 53. Strong thermal activity was detected at Mayon during early June through September, according to this MIROVA graph (Log Radiative Power) due to the presence of active lava flows on the SE, S, and E flanks. Courtesy of MIROVA.

Low activity was reported during much of April and May; gas-and-steam emissions rose 100-900 m above the crater and generally drifted in different directions. A total of 52 rockfall events and 18 volcanic earthquakes were detected during April and 147 rockfall events and 13 volcanic events during May. Sulfur dioxide flux measurements ranged between 400-576 tons per day (t/d) during April, the latter of which was measured on 29 April and between 162-343 t/d during May, the latter of which was measured on 13 May.

Activity during June increased, characterized by lava flows, pyroclastic density currents (PDCs), crater incandescence and incandescent rockfall events, gas-and-steam emissions, and continued seismicity. Weather clouds often prevented clear views of the summit, but during clear days, moderate gas-and-steam emissions rose 100-2,500 m above the crater and drifted in multiple directions. A total of 6,237 rockfall events and 288 volcanic earthquakes were detected. The rockfall events often deposited material on the S and SE flanks within 700-1,500 m of the summit crater and ash from the events drifted SW, S, SE, NE, and E. Sulfur dioxide emissions ranged between 149-1,205 t/d, the latter of which was measured on 10 June. Short-term observations from EDM and electronic tiltmeter monitoring indicated that the upper slopes were inflating since February 2023. Longer-term ground deformation parameters based on EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano remained inflated, especially on the NW and SE flanks. At 1000 on 5 June the Volcano Alert Level (VAL) was raised to 2 (on a 0-5 scale). PHIVOLCS noted that although low-level volcanic earthquakes, ground deformation, and volcanic gas emissions indicated unrest, the steep increase in rockfall frequency may indicate increased dome activity.

A total of 151 dome-collapse PDCs occurred during 8-9 and 11-30 June, traveled 500-2,000 m, and deposited material on the S flank within 2 km of the summit crater. During 8-9 June the VAL was raised to 3. At approximately 1947 on 11 June lava flow activity was reported; two lobes traveled within 500 m from the crater and deposited material on the S (Mi-isi), SE (Bonga), and E (Basud) flanks. Weak seismicity accompanied the lava flow and slight inflation on the upper flanks. This lava flow remained active through 30 June, moving down the S and SE flank as far as 2.5 km and 1.8 km, respectively and depositing material up to 3.3 km from the crater. During 15-16 June traces of ashfall from the PDCs were reported in Sitio Buga, Nabonton, City of Ligao and Purok, and San Francisco, Municipality of Guinobatan. During 28-29 June there were two PDCs generated by the collapse of the lava flow front, which generated a light-brown ash plume 1 km high. Satellite monitors detected significant concentrations of sulfur dioxide beginning on 29 June. On 30 June PDCs primarily affected the Basud Gully on the E flank, the largest of which occurred at 1301 and lasted eight minutes, based on the seismic record. Four PDCs generated between 1800 and 2000 that lasted approximately four minutes each traveled 3-4 km on the E flank and generated an ash plume that rose 1 km above the crater and drifted N and NW. Ashfall was recorded in Tabaco City.

Similar strong activity continued during July; slow lava effusion remained active on the S and SE flanks and traveled as far as 2.8 km and 2.8 km, respectively and material was deposited as far as 4 km from the crater. There was a total of 6,983 rockfall events and 189 PDCs that affected the S, SE, and E flanks. The volcano network detected a total of 2,124 volcanic earthquakes. Continuous gas-and-steam emissions rose 200-2,000 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 792-4,113 t/d, the latter of which was measured on 28 July. During 2-4 July three PDCs were generated from the collapse of the lava flow and resulting light brown plumes rose 200-300 m above the crater. Continuous tremor pulses were reported beginning at 1547 on 3 July through 7 July at 1200, at 2300 on 8 July and going through 0300 on 10 July, and at 2300 on 16 July, as recorded by the seismic network. During 6-9 July there were 10 lava flow-collapse-related PDCs that generated light brown plumes 300-500 m above the crater. During 10-11 July light ashfall was reported in some areas of Mabinit, Legazpi City, Budiao and Salvacion, Daraga, and Camalig, Albay. By 18 July the lava flow advanced 600 m on the E flank as well.

During 1733 on 18 July and 0434 on 19 July PHIVOLCS reported 30 “ashing” events, which are degassing events accompanied by audible thunder-like sounds and entrained ash at the crater, which produced short, dark plumes that drifted SW. These events each lasted 20-40 seconds, and plume heights ranged from 150-300 m above the crater, as recorded by seismic, infrasound, visual, and thermal monitors. Three more ashing events occurred during 19-20 July. Short-term observations from electronic tilt and GPS monitoring indicate deflation on the E lower flanks in early July and inflation on the NW middle flanks during the third week of July. Longer-term ground deformation parameters from EDM, precise leveling, continuous GPS, and electronic tilt monitoring indicated that the volcano was still generally inflated relative to baseline levels. A short-lived lava pulse lasted 28 seconds at 1956 on 21 July, which was accompanied by seismic and infrasound signals. By 22 July, the only lava flow that remained active was on the SE flank, and continued to extend 3.4 km, while those on the S and E flanks weakened markedly. One ashing event was detected during 30-31 July, whereas there were 57 detected during 31 July-1 August; according to PHIVOLCS beginning at approximately 1800 on 31 July eruptive activity was dominated by phases of intermittent ashing, as well as increased in the apparent rates of lava effusion from the summit crater. The ashing phases consisted of discrete events recorded as low-frequency volcanic earthquakes (LFVQ) typically 30 seconds in duration, based on seismic and infrasound signals. Gray ash plume rose 100 m above the crater and generally drifted NE. Shortly after these ashing events began, new lava began to effuse rapidly from the crater, feeding the established flowed on the SE, E, and E flanks and generating frequent rockfall events.

Intensified unrest persisted during August. There was a total of 4,141 rockfall events, 2,881 volcanic earthquakes, which included volcanic tremor events, 32 ashing events, and 101 PDCs detected throughout the month. On clear weather days, gas-and-steam emissions rose 300-1,500 m above the crater and drifted in different directions (figure 54). Sulfur dioxide emissions averaged 735-4,756 t/d, the higher value of which was measured on 16 August. During 1-2 August the rate of lava effusion decreased, but continued to feed the flows on the SE, S, and E flanks, maintaining their advances to 3.4 km, 2.8 km, and 1.1 km from the crater, respectively (figure 55). Rockfall and PDCs generated by collapses at the lava flow margins and from the summit dome deposited material within 4 km of the crater. During 3-4 August there were 10 tremor events detected that lasted 1-4 minutes. Short-lived lava pulse lasted 35 seconds and was accompanied by seismic and infrasound signals at 0442 on 6 August. Seven collapses were recorded at the front of the lava flow during 12-14 August.

Figure 54. Photo of Mayon showing a white gas-and-steam plume rising 800-1,500 m above the crater at 0645 on 25 August. Courtesy of William Rogers.

Figure 55. Photo of Mayon facing N showing incandescent lava flows and summit crater incandescence taken at 1830 on 25 August 2023. Courtesy of William Rogers.

During September, similar activity of slow lava effusion, PDCs, gas-and-steam emissions, and seismicity continued. There was a total of 4,452 rockfall events, 329 volcanic earthquakes, which included volcanic tremor events, two ashing events, and 85 PDCs recorded throughout the month. On clear weather days, gas-and-steam emissions rose 100-1,500 m above the crater and drifted in multiple directions. Sulfur dioxide emissions averaged 609-2,252 t/d, the higher average of which was measured on 6 September. Slow lava effusion continued advancing on the SE, S, and E flanks, maintaining lengths of 3.4 km, 2.8 km, and 1.1 km, respectively. Rockfall and PDC events generated by collapses along the lava flow margins and at the summit dome deposited material within 4 km of the crater.

Geologic Background. Symmetrical Mayon, which rises above the Albay Gulf NW of Legazpi City, is the most active volcano of the Philippines. The steep upper slopes are capped by a small summit crater. Recorded eruptions since 1616 CE range from Strombolian to basaltic Plinian, with cyclical activity beginning with basaltic eruptions, followed by longer periods of andesitic lava flows. Eruptions occur predominately from the central conduit and have also produced lava flows that travel far down the flanks. Pyroclastic density currents and mudflows have commonly swept down many of the approximately 40 ravines that radiate from the summit and have often damaged populated lowland areas. A violent eruption in 1814 killed more than 1,200 people and devastated several towns.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); William Rogers, Legazpi City, Albay Province, Philippines.

Nishinoshima, located about 1,000 km S of Tokyo, is a small island in the Ogasawara Arc in Japan. The island is the summit of a massive submarine volcano that has prominent submarine peaks to the S, W, and NE. Eruptions date back to 1973 and the current eruption period began in October 2022. Recent activity has consisted of small ash plumes and fumarolic activity (BGVN 48:07). This report covers activity during May through August 2023, using information from monthly reports of the Japan Meteorological Agency (JMA) monthly reports and satellite data.

Activity during May through June was relatively low. The Japan Coast Guard (JCG) did overflights on 14 and 22 June and reported white gas-and-steam emissions rising 600 m and 1,200 m from the central crater of the pyroclastic cone, respectively (figure 125). In addition, multiple white gas-and-steam emissions rose from the inner rim of the W side of the crater and from the SE flank of the pyroclastic cone. Discolored brown-to-green water was observed around almost the entire perimeter of the island; on 22 June light green discolored water was observed off the S coast of the island.

Figure 125. A white gas-and-steam plume rising 600 m above the crater of Nishinoshima at 1404 on 14 June 2023 (left) and 1,200 m above the crater at 1249 on 22 June 2023 (right). Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, June, 2023).

Observations from the Himawari meteorological satellite confirmed an eruption on 9 and 10 July. An eruption plume rose 1.6 km above the crater and drifted N around 1300 on 9 July. Satellite images acquired at 1420 and 2020 on 9 July and at 0220 on 10 July showed continuing emissions that rose 1.3-1.6 km above the crater and drifted NE and N. The Tokyo VAAC reported that an ash plume seen by a pilot and identified in a satellite image at 0630 on 21 July rose to 3 km altitude and drifted S.

Aerial observations conducted by JCG on 8 August showed a white-and-gray plume rising from the central crater of the pyroclastic cone, and multiple white gas-and-steam emissions were rising from the inner edge of the western crater and along the NW-SE flanks of the island (figure 126). Brown-to-green discolored water was also noted around the perimeter of the island.

Figure 126. Aerial photo of Nishinoshima showing a white-and-gray plume rising from the central crater taken at 1350 on 8 August 2023.

Intermittent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity), showing an increase in both frequency and power beginning in July (figure 127). This increase in activity coincides with eruptive activity on 9 and 10 July, characterized by eruption plumes. According to the MODVOLC thermal alert algorithm, one thermal hotspot was recorded on 20 July. Weak thermal anomalies were also detected in infrared satellite imagery, accompanied by strong gas-and-steam plumes (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Nishinoshima during May through August 2023, showing an increase in both frequency and power in July, according to this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 128. Infrared (bands B12, B11, B4) satellite images showing a small thermal anomaly at the crater of Nishinoshima on 30 June 2023 (top left), 3 July 2023 (top right), 7 August 2023 (bottom left), and 27 August 2023 (bottom right). Strong gas-and-steam plumes accompanied this activity, extending NW, NE, and SW. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Krakatau is located in the Sunda Strait between Java and Sumatra, Indonesia. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan cones and left only a remnant of Rakata. The post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones; it has been the site of frequent eruptions since 1927. The current eruption period began in May 2021 and has recently consisted of Strombolian eruptions and ash plumes (BGVN 48:07). This report describes lower levels of activity consisting of ash and white gas-and-steam plumes during May through August 2023, based on information provided by the Indonesian Center for Volcanology and Geological Hazard Mitigation, referred to as Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG), MAGMA Indonesia, and satellite data.

Activity was relatively low during May and June. Daily white gas-and-steam emissions rose 25-200 m above the crater and drifted in different directions. Five ash plumes were detected at 0519 on 10 May, 1241 on 11 May, 0920 on 12 May, 2320 on 12 May, and at 0710 on 13 May, and rose 1-2.5 km above the crater and drifted SW. A webcam image taken on 12 May showed ejection of incandescent material above the vent. A total of nine ash plumes were detected during 6-11 June: at 1434 and 00220 on 6 and 7 June the ash plumes rose 500 m above the crater and drifted NW, at 1537 on 8 June the ash plume rose 1 km above the crater and drifted SW, at 0746 and at 0846 on 9 June the ash plumes rose 800 m and 3 km above the crater and drifted SW, respectively, at 0423, 1431, and 1750 on 10 June the ash plumes rose 2 km, 1.5 km, and 3.5 km above the crater and drifted NW, respectively, and at 0030 on 11 June an ash plume rose 2 km above the crater and drifted NW. Webcam images taken on 10 and 11 June at 0455 and 0102, respectively, showed incandescent material ejected above the vent. On 19 June an ash plume at 0822 rose 1.5 km above the crater and drifted SE.

Similar low activity of white gas-and-steam emissions and few ash plumes were reported during July and August. Daily white gas-and-steam emissions rose 25-300 m above the crater and drifted in multiple directions. Three ash plumes were reported at 0843, 0851, and 0852 on 20 July that rose 500-2,000 m above the crater and drifted NW.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) graph of MODIS thermal anomaly data showed intermittent low-to-moderate power thermal anomalies during May through August 2023 (figure 140). Although activity was often obscured by weather clouds, a thermal anomaly was visible in an infrared satellite image of the crater on 12 May, accompanied by an eruption plume that drifted SW (figure 141).

Figure 140. Intermittent low-to-moderate power thermal anomalies were detected at Krakatau during May through August 2023, based on this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 141. A single thermal anomaly (bright yellow-orange) was visible at Krakatau in this infrared (bands B12, B11, B4) satellite image taken on 12 May 2023. An eruption plume accompanied the thermal anomaly and drifted SW. Courtesy of Copernicus Browser.

Geologic Background. The renowned Krakatau (frequently mis-named as Krakatoa) volcano lies in the Sunda Strait between Java and Sumatra. Collapse of an older edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently the Rakata, Danan, and Perbuwatan cones were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones. Anak Krakatau has been the site of frequent eruptions since 1927.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Villarrica, in central Chile, consists of a 2-km-wide caldera that formed about 3,500 years ago and is located at the base of the presently active cone at the NW margin of a 6-km-wide caldera. Historical eruptions eruptions date back to 1558 and have been characterized by mild-to-moderate explosive activity with occasional lava effusions. The current eruption period began in December 2014 and has recently consisted of nighttime crater incandescence, ash emissions, and seismicity (BGVN 48:04). This report covers activity during April through September 2023 and describes occasional Strombolian activity, gas-and-ash emissions, and nighttime crater incandescence. Information for this report primarily comes from the Southern Andes Volcano Observatory (Observatorio Volcanológico de Los Andes del Sur, OVDAS), part of Chile's National Service of Geology and Mining (Servicio Nacional de Geología y Minería, SERNAGEOMIN) and satellite data.

Seismicity during April consisted of long period (LP) events and tremor (TRE); a total of 9,413 LP-type events and 759 TR-type events were detected throughout the month. Nighttime crater incandescence persisted and was visible in the degassing column. Sulfur dioxide data was obtained using Differential Absorption Optical Spectroscopy Equipment (DOAS) that showed an average value of 1,450 ± 198 tons per day (t/d) during 1-15 April and 1,129 ± 201 t/d during 16-30 April, with a maximum daily value of 2,784 t/d on 9 April. Gas-and-steam emissions of variable intensities rose above the active crater as high as 1.3 km above the crater on 13 April. Strombolian explosions were not observed and there was a slight decrease in the lava lake level.

There were 14,123 LP-type events and 727 TR-type events detected during May. According to sulfur dioxide measurements taken with DOAS equipment, the active crater emitted an average value of 1,826 ± 482 t/d during 1-15 May and 912 ± 41 t/d during 16-30 May, with a daily maximum value of 5,155 t/d on 13 May. Surveillance cameras showed continuous white gas-and-steam emissions that rose as high as 430 m above the crater on 27 May. Nighttime incandescence illuminated the gas column less than 300 m above the crater rim was and no pyroclastic emissions were reported. A landslide was identified on 13 May on the E flank of the volcano 50 m from the crater rim and extending 300 m away; SERNAGEOMIN noted that this event may have occurred on 12 May. During the morning of 27 and 28 May minor Strombolian explosions characterized by incandescent ejecta were recorded at the crater rim; the last reported Strombolian explosions had occurred at the end of March.

Seismic activity during June consisted of five volcano-tectonic (VT)-type events, 21,606 LP-type events, and 2,085 TR-type events. The average value of sulfur dioxide flux obtained by DOAS equipment was 1,420 ± 217 t/d during 1-15 June and 2,562 ± 804 t/d, with a maximum daily value of 4,810 t/d on 17 June. White gas-and-steam emissions rose less than 480 m above the crater; frequent nighttime crater incandescence was reflected in the degassing plume. On 12 June an emission rose 100 m above the crater and drifted NNW. On 15 June one or several emissions resulted in ashfall to the NE as far as 5.5 km from the crater, based on a Skysat satellite image. Several Strombolian explosions occurred within the crater; activity on 15 June was higher energy and ejected blocks 200-300 m on the NE slope. Surveillance cameras showed white gas-and-steam emissions rising 480 m above the crater on 16 June. On 19 and 24 June low-intensity Strombolian activity was observed, ejecting material as far as 200 m from the center of the crater to the E.

During July, seismicity included 29,319 LP-type events, 3,736 TR-type events, and two VT-type events. DOAS equipment recorded two days of sulfur dioxide emissions of 4,220 t/d and 1,009 t/d on 1 and 13 July, respectively. Constant nighttime incandescence was also recorded and was particularly noticeable when accompanied by eruptive columns on 12 and 16 July. Minor explosive events were detected in the crater. According to Skysat satellite images taken on 12, 13, and 16 July, ashfall deposits were identified 155 m S of the crater. According to POVI, incandescence was visible from two vents on the crater floor around 0336 on 12 July. Gas-and-ash emissions rose as high as 1.2 km above the crater on 13 July and drifted E and NW. A series of gas-and-steam pulses containing some ash deposited material on the upper E flank around 1551 on 13 July. During 16-31 July, average sulfur dioxide emissions of 1,679 ± 406 t/d were recorded, with a maximum daily value of 2,343 t/d on 28 July. Fine ash emissions were also reported on 16, 17, and 23 July.

Seismicity persisted during August, characterized by 27,011 LP-type events, 3,323 TR-type events, and three VT-type events. The average value of sulfur dioxide measurements taken during 1-15 August was 1,642 ± 270 t/d and 2,207 ± 4,549 t/d during 16-31 August, with a maximum daily value of 3,294 t/d on 27 August. Nighttime crater incandescence remained visible in degassing columns. White gas-and-steam emissions rose 480 m above the crater on 6 August. According to a Skysat satellite image from 6 August, ash accumulation was observed proximal to the crater and was mainly distributed toward the E slope. White gas-and-steam emissions rose 320 m above the crater on 26 August. Nighttime incandescence and Strombolian activity that generated ash emissions were reported on 27 August.

Seismicity during September was characterized by five VT-type events, 12,057 LP-type events, and 2,058 TR-type events. Nighttime incandescence persisted. On 2 September an ash emission rose 180 m above the crater and drifted SE at 1643 (figure 125) and a white gas-and-steam plume rose 320 m above the crater. According to the Buenos Aires VAAC, periods of continuous gas-and-ash emissions were visible in webcam images from 1830 on 2 September to 0110 on 3 September. Strombolian activity was observed on 2 September and during the early morning of 3 September, the latter event of which generated an ash emission that rose 60 m above the crater and drifted 100 m from the center of the crater to the NE and SW. Ashfall was reported to the SE and S as far as 750 m from the crater. The lava lake was active during 3-4 September and lava fountaining was visible for the first time since 26 March 2023, according to POVI. Fountains captured in webcam images at 2133 on 3 September and at 0054 on 4 September rose as high as 60 m above the crater rim and ejected material onto the upper W flank. Sulfur dioxide flux of 1,730 t/d and 1,281 t/d was measured on 3 and 4 September, respectively, according to data obtained by DOAS equipment.

Figure 125. Webcam image of a gray ash emission rising above Villarrica on 2 September 2023 at 1643 (local time) that rose 180 m above the crater and drifted SE. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 02 de septiembre de 2023, 17:05 Hora local).

Strong Strombolian activity and larger gas-and-ash plumes were reported during 18-20 September. On 18 September activity was also associated with energetic LP-type events and notable sulfur dioxide fluxes (as high as 4,277 t/d). On 19 September Strombolian activity and incandescence were observed. On 20 September at 0914 ash emissions rose 50 m above the crater and drifted SSE, accompanied by Strombolian activity that ejected material less than 100 m SSE, causing fall deposits on that respective flank. SERNAGEOMIN reported that a Planet Scope satellite image taken on 20 September showed the lava lake in the crater, measuring 32 m x 35 m and an area of 0.001 km². Several ash emissions were recorded at 0841, 0910, 1251, 1306, 1312, 1315, and 1324 on 23 September and rose less than 150 m above the crater. The sulfur dioxide flux value was 698 t/d on 23 September and 1,097 t/d on 24 September. On 24 September the Volcanic Alert Level (VAL) was raised to Orange (the third level on a four-color scale). SENAPRED maintained the Alert Level at Yellow (the middle level on a three-color scale) for the communities of Villarrica, Pucón (16 km N), Curarrehue, and Panguipulli.

During 24-25 September there was an increase in seismic energy (observed at TR-events) and acoustic signals, characterized by 1 VT-type event, 213 LP-type events, and 124 TR-type events. Mainly white gas-and-steam emissions, in addition to occasional fine ash emissions were recorded. During the early morning of 25 September Strombolian explosions were reported and ejected material 250 m in all directions, though dominantly toward the NW. On 25 September the average value of sulfur dioxide flux was 760 t/d. Seismicity during 25-30 September consisted of five VT-type events, 1,937 LP-type events, and 456 TR-type events.

During 25-29 September moderate Strombolian activity was observed and ejected material as far as the crater rim. In addition, ash pulses lasting roughly 50 minutes were observed around 0700 and dispersed ENE. During 26-27 September a TR episode lasted 6.5 hours and was accompanied by discrete acoustic signals. Satellite images from 26 September showed a spatter cone on the crater floor with one vent that measured 10 x 14 m and a smaller vent about 35 m NE of the cone. SERNAGEOMIN reported an abundant number of bomb-sized blocks up to 150 m from the crater, as well as impact marks on the snow, which indicated explosive activity. A low-altitude ash emission was observed drifting NW around 1140 on 28 September, based on webcam images. Between 0620 and 0850 on 29 September an ash emission rose 60 m above the crater and drifted NW. During an overflight taken around 1000 on 29 September scientists observed molten material in the vent, a large accumulation of pyroclasts inside the crater, and energetic degassing, some of which contained a small amount of ash. Block-sized pyroclasts were deposited on the internal walls and near the crater, and a distal ash deposit was also visible. The average sulfur dioxide flux measured on 28 September was 344 t/d. Satellite images taken on 29 September ashfall was deposited roughly 3 km WNW from the crater and nighttime crater incandescence remained visible. The average sulfur dioxide flux value from 29 September was 199 t/d. On 30 September at 0740 a pulsating ash emission rose 1.1 km above the crater and drifted NNW (figure 126). Deposits on the S flank extended as far as 4.5 km from the crater rim, based on satellite images from 30 September.

Figure 126. Webcam image of a gray ash plume rising 1.1 km above the crater of Villarrica at 0740 (local time) on 30 September 2023. Courtesy of SERNAGEOMIN (Reporte Especial de Actividad Volcanica (REAV), Region De La Araucania y Los Rios, Volcan Villarrica, 30 de septiembre de 2023, 09:30 Hora local).

Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) showed intermittent thermal activity during April through September, with slightly stronger activity detected during late September (figure 127). Small clusters of thermal activity were detected during mid-June, early July, early August, and late September. According to the MODVOLC thermal alert system, a total of four thermal hotspots were detected on 7 July and 3 and 23 September. This activity was also intermittently captured in infrared satellite imagery on clear weather days (figure 128).

Figure 127. Low-to-moderate power thermal anomalies were detected at Villarrica during April through September 2023, according to this MIROVA graph (Log Radiative Power). Activity was relatively low during April through mid-June. Small clusters of activity occurred during mid-June, early July, early August, and late September. Courtesy of MIROVA.

Figure 128. Consistent bright thermal anomalies (bright yellow-orange) were visible at the summit crater of Villarrica in infrared (bands B12, B11, B4) satellite images, as shown on 17 June 2023 (top left), 17 July 2023 (top right), 6 August 2023 (bottom left), and 20 September 2023 (bottom right). Courtesy of Copernicus Browser.

Geologic Background. The glacier-covered Villarrica stratovolcano, in the northern Lakes District of central Chile, is ~15 km south of the city of Pucon. A 2-km-wide caldera that formed about 3,500 years ago is located at the base of the presently active, dominantly basaltic to basaltic andesite cone at the NW margin of a 6-km-wide Pleistocene caldera. More than 30 scoria cones and fissure vents are present on the flanks. Plinian eruptions and pyroclastic flows that have extended up to 20 km from the volcano were produced during the Holocene. Lava flows up to 18 km long have issued from summit and flank vents. Eruptions documented since 1558 CE have consisted largely of mild-to-moderate explosive activity with occasional lava effusion. Glaciers cover 40 km2 of the volcano, and lahars have damaged towns on its flanks.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/); Proyecto Observación Villarrica Internet (POVI) (URL: http://www.povi.cl/); Sistema y Servicio Nacional de Prevención y Repuesta Ante Desastres (SENAPRED), Av. Beauchef 1671, Santiago, Chile (URL: https://web.senapred.cl/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Merapi, located just north of the major city of Yogyakarta in central Java, Indonesia, has had activity within the last 20 years characterized by pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome. The current eruption period began in late December 2020 and has more recently consisted of ash plumes, intermittent incandescent avalanches of material, and pyroclastic flows (BGVN 48:04). This report covers activity during April through September 2023, based on information from Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), the Center for Research and Development of Geological Disaster Technology, a branch of PVMBG which specifically monitors Merapi. Additional information comes from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), MAGMA Indonesia, the Darwin Volcanic Ash Advisory Centre (VAAC), and various satellite data.

Activity during April through September 2023 primarily consisted of incandescent avalanches of material that mainly affected the SW and W flanks and traveled as far as 2.3 km from the summit (table 25) and white gas-and-steam emissions that rose 10-1,000 m above the crater.

Table 25. Monthly summary of avalanches and avalanche distances recorded at Merapi during April through September 2023. The number of reported avalanches does not include instances where possible avalanches were heard but could not be visually confirmed as a result of inclement weather. Data courtesy of BPPTKG (April-September 2023 daily reports).

BPPTKG reported that during April and May white gas-and-steam emissions rose 10-750 m above the crater, incandescent avalanches descended 500-2,000 m on the SW and W flanks (figure 135). Cloudy weather often prevented clear views of the summit, and sometimes avalanches could not be confirmed. According to a webcam image, a pyroclastic flow was visible on 17 April at 0531. During the week of 28 April and 4 May a pyroclastic flow was reported on the SW flank, traveling up to 2.5 km. According to a drone overflight taken on 17 May the SW lava dome volume was an estimated 2,372,800 cubic meters and the dome in the main crater was an estimated 2,337,300 cubic meters.

Figure 135. Photo showing an incandescent avalanche affecting the flank of Merapi on 8 April 2023. Courtesy of Øystein Lund Andersen.

During June and July similar activity persisted with white gas-and-steam emissions rising 10-350 m above the crater and frequent incandescent avalanches that traveled 300-2,000 m down the SW, W, and S flanks (figure 136). Based on an analysis of aerial photos taken on 24 June the volume of the SW lava dome was approximately 2.5 million cubic meters. A pyroclastic flow was observed on 5 July that traveled 2.7 km on the SW flank. According to the Darwin VAAC multiple minor ash plumes were identified in satellite images on 19 July that rose to 3.7 km altitude and drifted S and SW. During 22, 25, and 26 July a total of 17 avalanches descended as far as 1.8 km on the S flank.

Figure 136. Photo showing an incandescent avalanche descending the flank of Merapi on 23 July 2023. Courtesy of Øystein Lund Andersen.

Frequent white gas-and-steam emissions continued during August and September, rising 10-450 m above the crater. Incandescent avalanches mainly affected the SW and W flanks and traveled 400-2,300 m from the vent (figure 137). An aerial survey conducted on 10 August was analyzed and reported that estimates of the SW dome volume was 2,764,300 cubic meters and the dome in the main crater was 2,369,800 cubic meters.

Figure 137. Photo showing a strong incandescent avalanche descending the flank of Merapi on 23 September 2023. Courtesy of Øystein Lund Andersen.

Frequent and moderate-power thermal activity continued throughout the reporting period, according to a MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data (figure 138). There was an increase in the number of detected anomalies during mid-May. The MODVOLC thermal algorithm recorded a total of 47 thermal hotspots: six during April, nine during May, eight during June, 15 during July, four during August, and five during September. Some of this activity was captured in infrared satellite imagery on clear weather days, sometimes accompanied by incandescent material on the SW flank (figure 139).

Figure 138. Frequent and moderate-power thermal anomalies were detected at Merapi during April through September 2023, as shown on this MIROVA plot (Log Radiative Power). There was an increase in the number of anomalies recorded during mid-May. Courtesy of MIROVA.

Figure 139. Infrared (bands B12, B11, B4) satellite images showed a consistent thermal anomaly (bright yellow-orange) at the summit crater of Merapi on 8 April 2023 (top left), 18 May 2023 (top right), 17 June 2023 (middle left), 17 July 2023 (middle right), 11 August 2023 (bottom left), and 20 September 2023 (bottom right). Incandescent material was occasionally visible descending the SW flank, as shown in each of these images. Courtesy of Copernicus Browser.

Geologic Background. Merapi, one of Indonesia's most active volcanoes, lies in one of the world's most densely populated areas and dominates the landscape immediately north of the major city of Yogyakarta. It is the youngest and southernmost of a volcanic chain extending NNW to Ungaran volcano. Growth of Old Merapi during the Pleistocene ended with major edifice collapse perhaps about 2,000 years ago, leaving a large arcuate scarp cutting the eroded older Batulawang volcano. Subsequent growth of the steep-sided Young Merapi edifice, its upper part unvegetated due to frequent activity, began SW of the earlier collapse scarp. Pyroclastic flows and lahars accompanying growth and collapse of the steep-sided active summit lava dome have devastated cultivated lands on the western-to-southern flanks and caused many fatalities.

Information Contacts: Balai Penyelidikan dan Pengembangan Teknologi Kebencanaan Geologi (BPPTKG), Center for Research and Development of Geological Disaster Technology (URL: http://merapi.bgl.esdm.go.id/, Twitter: @BPPTKG); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Øystein Lund Andersen (URL: https://www.oysteinlundandersen.com/, https://twitter.com/oysteinvolcano).

Ebeko, located on the N end of Paramushir Island in Russia’s Kuril Islands just S of the Kamchatka Peninsula, consists of three summit craters along a SSW-NNE line at the northern end of a complex of five volcanic cones. Observed eruptions date back to the late 18th century and have been characterized as small-to-moderate explosions from the summit crater, accompanied by intense fumarolic activity. The current eruptive period began in June 2022, consisting of frequent explosions, ash plumes, and thermal activity (BGVN 47:10, 48:06). This report covers similar activity during June-November 2023, based on information from the Kamchatka Volcanic Eruptions Response Team (KVERT) and satellite data.

Moderate explosive activity continued during June-November 2023 (figures 50 and 51). According to visual data from Severo-Kurilsk, explosions sent ash 2-3.5 km above the summit (3-4.5 km altitude) during most days during June through mid-September. Activity after mid-September was slightly weaker, with ash usually reaching less than 2 km above the summit. According to KVERT the volcano in October and November was, with a few exceptions, either quiet or obscured by clouds that prevented satellite observations. KVERT issued Volcano Observatory Notices for Aviation (VONA) on 8 and 12 June, 13 and 22 July, 3 and 21 August, and 31 October warning of potential aviation hazards from ash plumes drifting 3-15 km from the volcano. Based on satellite data, KVERT reported a persistent thermal anomaly whenever weather clouds permitted viewing.

Figure 50. Ash explosion from the active summit crater of Ebeko on 18 July 2023; view is approximately towards the W. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Figure 51. Ash explosion from the active summit crater of Ebeko on 23 July 2023 with lightning visible in the lower part of the plume. Photo provided by I. Bolshakov and M.V. Lomonosov MGU; courtesy of KVERT.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/).

Eruptive activity has been ongoing at Bagana since February 2000, and frequently active for over 150 years. Due to the remote location of this volcano, the most reliable observations of activity come from the identification of ash plumes in satellite imagery by the Darwin Volcanic Ash Advisory Centre (VAAC) and thermal anomalies from satellite infrared sensors.

Since July 2016 (BGVN 41:07), the Darwin VAAC issued aviation warnings of ash plumes almost every week through mid-June 2017. The plumes typically rose to between 1.8 and 3.4 km; the most commonly reported altitude of the plume was about 2.1 km. The plumes drifted in multiple directions depending on the local wind patterns. Drift directions were not always reported, but a few reached 110-120 km, and one was observed as far as 160 km away on 7 September 2016.

MODIS data processed by the MIROVA algorithm (figure 20) reinforce the Darwin VAAC reports of a nearly continuous eruption since July 2016 through mid-June 2017. Frequent MODVOLC thermal alerts, also based on MODIS satellite-based data, corroborate the MIROVA analysis.

Figure 20. Thermal anomalies at Bagana shown on a MIROVA plot (Log Radiative Power) for the year ending 12 June 2017. Courtesy of MIROVA.

Geologic Background. Bagana volcano, in a remote portion of central Bougainville Island, is frequently active. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although occasional explosive activity produces pyroclastic flows. Lava flows with tongue-shaped lobes up to 50 m thick and prominent levees descend the flanks on all sides.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

Activity at Bulusan typically has included phreatic explosions from the summit crater and flank vents, ash-and-steam plumes, and minor ashfall in nearby villages (BGVN 41:03, 42:02). The danger zone was expanded in October 2016 when a fissure extended 2 km down the upper S flank that was the source of multiple phreatic explosions (figure 10, and see BGVN 42:02). During the first eight months of 2017, eruptive activity included similar episodes on 2 March and 5 June. Information was provided by the Philippine Institute of Volcanology and Seismology (PHIVOLCS). Throughout the reporting period of 1 January-8 September 2017 the Alert Level remained at 1, indicating a low level of volcanic unrest and a 4-km-radius Permanent Danger Zone (PDZ).

Figure 10. A phreatic ash explosion from the SE vent at Bulusan on 17 October 2016 lasted 24 minutes. White steam plumes can be seen rising from other vents. Photo by Drew Zuñiga and provided by 2D Albay, as published in The Philippine Star (18 October 2016).

According to PHIVOLCS, a weak phreatic eruption occurred at 1357 on 2 March 2017. The event was recorded by the seismic network as an explosion-type earthquake followed by short-duration tremor that lasted approximately 26 minutes. Visual observations were obscured by weather clouds, although a small steam plume rising from the SE vent was recorded by a webcam.

On 5 June 2017 another weak phreatic eruption was recorded at 1029 by the seismic network for 12 minutes. The eruption again could not be visually observed due to dense weather clouds covering the summit. Minor ashfall, a sulfuric odor, and a rumbling sound were reported in the barangays (neighborhoods) of Monbon and Cogon, while sulfuric odor was noted in the barangay of Bolos. These three neighborhoods are within the municipality of Irosin, about 8 km SSW of the volcano.

According to a news account (Manila Bulletin), precise leveling data obtained during 29 January to 3 February 2017 indicated deflationary changes since October 2016. PHIVOLCS reported that precise leveling data obtained during 14-23 June 2017 indicated inflation since February 2017. According to PHIVOLCS, continuous GPS measurements have indicated an inflationary trend since July 2016.

Sulfur dioxide emissions on 31 July and 20 August, reported by PHIVOLCS, averaged 82 tonnes/day, which according to a news account (Manila Bulletin) was the same as measured on 29 April 2017. The seismic monitoring network recorded three volcanic earthquakes on 7-8 September. Weak steam plumes from the active vents rose to 50 meters and drifted SW.

Geologic Background. Luzon's southernmost volcano, Bulusan, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera, which was formed about 36,000 years ago. It lies at the SE end of the Bicol volcanic arc occupying the peninsula of the same name that forms the elongated SE tip of Luzon. A broad, flat moat is located below the topographically prominent SW rim of Irosin caldera; the NE rim is buried by the andesitic complex. Bulusan is flanked by several other large intracaldera lava domes and cones, including the prominent Mount Jormajan lava dome on the SW flank and Sharp Peak to the NE. The summit is unvegetated and contains a 300-m-wide, 50-m-deep crater. Three small craters are located on the SE flank. Many moderate explosive eruptions have been recorded since the mid-19th century.

Information Contacts: Philippine Institute of Volcanology and Seismology (PHIVOLCS), Department of Science and Technology, University of the Philippines Campus, Diliman, Quezon City, Philippines (URL: http://www.phivolcs.dost.gov.ph/); Manila Bulletin (URL: http://mb.com.ph/); Philippine Star (URL: http://www.philstar.com/).

Frequent historical eruptions at México's Volcán de Colima (Volcán Fuego) date back to the 16th century and include vulcanian and phreatic explosions, lava flows, large debris avalanches, and pyroclastic flows. The latest eruptive episode began in January 2013. Extensive activity in 2015 included near-constant ash plumes with extensive ashfall, lava flows, and pyroclastic flows (BGVN 41:01). The eruption continued throughout 2016 until the last ash-bearing explosion was reported on 7 March 2017. This report covers the activity through June 2017. Most of the information for this report was gathered from the Unidad Estatal de Protección Civil de Colima (UEPCC), the Centro Universitario de Estudios e Investigaciones de Vulcanologia, Universidad de Colima (CUEIV-UdC), and the Washington Volcanic Ash Advisory Center (VAAC).

Colima was very active from January through April 2016 with hundreds of ash emissions, and a slow-growing lava dome that was first observed on 19 February. Activity decreased during May-September, although multiple explosions with ash plumes still took place most weeks during the period. On 30 September, the lava dome overflowed the crater rim, and sent a slow-moving lava flow and incandescent material down the SW flank. The lava flow continued to grow, reaching over 2 km in length by the end of October. A second lava flow appeared in mid-November, and advanced 1.7 km by early December. Strong ash-bearing explosions during December 2016-January 2017 sent plumes to heights of 4-6 km above the crater. Activity decreased during the second half of February; the last ash-bearing explosion was reported on 7 March 2017. Decreasing seismicity and minor landslides were reported through June 2017 with no further eruptive activity.

Incandescent activity during explosions in January 2016 sent glowing blocks down the flanks of Colima along with spectacular lightning in the ash plumes (figure 119). Ash emissions continued at Colima at a very high rate of multiple daily events, similar to December 2015 (figure 120). The Washington VAAC issued multiple advisories nearly every day during the month with information based on satellite imagery, wind data, webcam images, and notices from the México City Meteorological Watch Office (MWO). The ash plumes rose to altitudes of 4.3-6.7 km and most commonly drifted N or E. They generally drifted a few tens of kilometers before dissipating, but a few were still visible as far as 200 km from the summit.

Figure 119. Eruption of ash plume and incandescent material at Colima on 3 January 2016. Courtesy of Volcano Discovery.

Figure 120. Ash eruption at Colima on 10 January 2016. Image from the Webcams de México Colima webcam located at the Laguna de Carrizalillos in Comala, about 25 km SW of the summit.

Multiple daily ash advisories from the Washington VAAC continued during 1-9 February. They resumed on 14 February, and were intermittent for the rest of the month with similar altitudes and drift directions as those observed during January, but at a slightly lower frequency, decreasing towards the end of the month. On 19 February, CUEIV-UdC researcher Nick Varley observed a lava dome emerging from the floor of the crater (figure 121) during a helicopter overflight. It was estimated to be 25-30 m in diameter and 10 m high inside the almost 300-m-diameter, 50-m-deep summit crater. By 29 February, the dome had increased in size (figure 122), and fumarolic activity had also increased on the SE side of the summit crater.

Figure 121. A new lava dome in the summit crater of Colima on 19 February 2016. Courtesy of CUEIV-UdC (http://www.ucol.mx/enterate/nota.php?docto=2473).

Figure 122. The lava dome at Colima photographed on 29 February 2016 was noticeably larger than when first photographed ten days earlier. Courtesy of SkyAlert (2 March 2016).

Ash plume heights during March 2016 were slightly lower than during February (4.0-6.1 km altitude). Most of the plumes continued to drift NE or SE, and most dissipated within 50 km. During the first week of April, scientists observed fresh ashfall covering the dome at the center of the crater, which had not changed significantly since the previous overflight at the end of February. Persistent ash plumes continued throughout April with a three-minute-long ash emission recorded on 28 April by Colima's webcam.

The frequency of ash emissions decreased during May 2016 and further still during June 2016, when advisories from the Washington VAAC only appeared during five days of the month (1, 4, 13, 23, 30); the plume heights remained similar to previous months, except for a 16 May plume observed moving ENE at 7.6 km. After a two week pause, ash emissions resumed on 17 July with plume heights ranging from 4.3 to 7.3 km altitude through the end of the month. During the second half of August and for part of September, intermittent plumes did not exceed 6.1 km altitude, and dissipated within a few tens of kilometers of the summit.

The Unidad Estatal de Protección Civil de Colima reported that on 26 September seismicity at Colima increased, and incandescence appeared at the crater. On 27 September, small landslides originating from the growing lava dome traveled 100 m down the S flank. By the evening of 30 September, the webcam showed intense activity and crater incandescence as lava spilled over the crater rim and flowed down the SW flank (figure 123). An intense thermal anomaly was visible in short-wave infrared satellite images. An ash plume detected on 1 October in satellite images drifted almost 40 km S and SW; the webcam recorded explosions and pyroclastic flows down the flanks. The OMI instrument on the Aura satellite also recorded significant SO₂ plumes drifting W and SW from Colima on 30 September and 1 October (figure 124).

Figure 123. Intense activity at Colima during the late evening of 30 September 2016 (2014 CST), as a new lava flow emerged from the summit crater and moved down the SW flank. Image from the Webcams de México Colima webcam located at the Laguna de Carrizalillos in Comala, about 25 km SW of the summit.

Figure 124. Sulfur dioxide plumes from Colima were captured by the OMI instrument on the Aura satellite on 30 September (upper) and 1 October 2016 (lower). Colima is on the left (west) side, near the coast. The other SO2 plume in central Mexico on the 1 October is from Popocatépetl. The red pixels indicate Dobson Unit (DU) values greater than 2. DU are a measure of molecular density of SO2 in the atmosphere. Courtesy of NASA Goddard Space Flight Center.

According to news articles (Noticieros Televisa), during 29 September-1 October gas-and-ash plumes rose 4 km and caused ashfall in nearby areas, including La Becererra, La Yerbabuena, San Antonio, and El Jabali in the municipality of Comala (26 km SW), Montitlán in the municipality of Cuauhtémoc (34 km NW), and Juan Barragan in Tonila, Jalisco (14 km SE). On 1 October the Colima State government stated that the communities of La Yerbabuena (80 people) and La Becerrera (230 people) were preemptively evacuated, and an exclusion zone was extended to 12 km on the SW side. A news article noted that Juan Barragan was also evacuated.

The lava flow continued down the flank with incandescent rockfalls (figure 125) and occasional pyroclastic flows; by 4 October it had reached the base of the cone. The volume of the lava dome was estimated to have exceeded 1.2 million cubic meters (figure 126). By 8 October 2016, the lava flow was about 2,000 m long and 270 m wide at its front at the base of the cone. The Washington VAAC reported a strong hotspot consistent with the lava flow in satellite imagery on 9 October. On 13 October, they noted an ash plume that had drifted over 200 km W from the summit. Strong, multi-pixel, daily thermal alerts were issued from MODVOLC during 1-14 October. On 21 October, UEPCC reported that lava continued to flow down the S flank. It was 2.3 km long, 320 m wide, and had an estimated volume of 21 million m³.

Figure 125. A lava flow descends the S flank of Colima on 2 October 2016. Image by Raúl Arámbula, courtesy of Red Sismologica Telemetrica del Estado de Colima-Centro Universitario de Estudios e Investigaciones de Vulcanologia-Universidad de Colima (RESCO-CUEIV-UdeC).

Figure 126. The lava dome overflowing the summit crater at Colima on 5 October 2016. Image by Raúl Arámbula, courtesy of RESCO-CUIEV-UdeC.

Multiple ash plumes rose to altitudes of 5.5-8.2 km and drifted 25-40 km S, SW, and W during 2-4 October. Ashfall was reported in areas on the S and SW flanks. Ash explosions were also frequent throughout the rest of October, with plumes rising to altitudes of 4.3-7 km on many days (figure 127), until they ceased on 4 November for several weeks.

Figure 127. Ash explosion at Colima on 9 October 2016. Steam in the foreground is from the lava flow travelling down the SW flank. Image from the Webcams de México Colima webcam located at the Laguna de Carrizalillos in Comala, about 25 km SW of the summit.

Effusive activity increased again at the very end of October 2016 with the growth of a new lava dome inside the summit crater. By 17 November, a new lava flow was also visible on the S flank (figure 128); it was reported to be about 500 m long by 20 November. After intermittent MODVOLC thermal alerts during late October and early November, they intensified with daily multi-pixel alerts between 15 November and 1 December.

Figure 128. A new lava flow on the S flank of Colima on 17 November 2016. Image from the Webcams de México Colima webcam located at the Laguna de Carrizalillos in Comala, about 25 km SW of the summit.

During 26-28 November 2016, a brief episode of ash emissions sent plumes to 4.9-5.5 km altitude that drifted W, N, and NE as far as 75 km before dissipating. Observations of Colima made on 5 December by UEPCC during a helicopter overflight indicated that the lava flow on the S flank was slowing its advance, and had reached about 1,700 m in length (figure 129).

Figure 129. The lava flow on the S flank of Colima had reached 1.7 km in length on 5 December 2016. Courtesy of UEPCC.

A new series of strong explosions with abundant ash emissions began on 7 December that continued through the end of the month. Multiple daily ash emissions appeared in both the webcam and satellite imagery. The plume on 8 December rose to 7.3 km and extended about 185 km NE of the summit near Lago de Chapala before dissipating. Incandescence during the explosions was visible at night, and glowing blocks were common on the upper flanks.

Ash clouds from multiple emissions were observed drifting W to WSW on 14 December at altitudes from 6.1 to 7.9 km (about 4 km above the summit). These plumes were visible 370 km WSW of the summit the next day. Plumes rose as high as 9.1 km altitude on 15 December, and spread N and NW. A series of strong, multiple daily explosions during 16-18 December included some of the strongest explosions since July 2015 (figure 130). Many of the multiple daily explosions during 19-31 December had plumes rising over 7 km in altitude and drifting over 100 km from the summit before dissipating. MODVOLC thermal alerts appeared on 13 days during December 2016.

Figure 130. A strong explosion at Colima on 18 December 2016. Image from the Webcams de México Colima webcam located at the Laguna de Carrizalillos in Comala, about 25 km SW of the summit.

Frequent strong explosive activity continued during January 2017. For the first three weeks of the month, the multiple daily plumes rose to altitudes of 4.6-7.6 km, drifting in multiple directions, some as far as 135 km. The UEPCC reported that at 0027 on 18 January a moderate-to-large explosion ejected incandescent material as far as 2 km onto the W, SW, SE, and N flanks. Based on webcam and satellite images, the México City MWO, and pilot observations, the Washington VAAC reported that during 18-24 January ash plumes rose to altitudes of 4.1-6.7 km and drifted in multiple directions. On 19 January, strong explosions were recorded by the webcam and noted by the Jalisco Civil Protection Agency (figure 131); they also reported ashfall in Comala and Cuauhtémoc. A strong thermal anomaly was identified in satellite images. Remnant ash clouds from the explosions were centered about 350 km SE on 20 January. A large ash plume rose to an altitude of 10.7 km on 23 January and drifted NE; several plumes that rose to over 7 km altitude were reported through the end of January. MODVOLC thermal alerts were issued on 11 days during January, but no further alerts appeared through June 2017.

Figure 131. Eruption at Colima at 0431 on 19 January 2017. Courtesy of Sergio Tapiro.

The CUEIV-UdC reported that a large explosion at 1732 on 3 February 2017 generated an ash plume that rose 6 km above the crater rim and drifted SSW (figure 132). The Washington VAAC reported the plume at 7.6 km altitude (3.7 km above the crater) shortly before midnight on 4 February. The CUEIV-UdC also noted that a small pyroclastic flow traveled down the E flank. Their report stated that the internal crater was about 250 m in diameter and 50-60 m deep; previous lava domes had been destroyed in late September and mid-November 2016.

Figure 132. An explosion at Colima on 3 February 2017 caused an ash plume that the Universidad de Colima reported as rising to six km above the crater, drifting SSW. A small pyroclastic flow descended the E flank. Image from the Webcams de México Colima webcam located at the Laguna de Carrizalillos in Comala, about 25 km SW of the summit.

A brief period of low-intensity explosions during 10-16 February 2017 generated ash plumes reported by the Washington VAAC at 4-5.2 km altitude. There were no further aviation alerts issued during February. According to CUEIV-UdC, a few low-intensity explosions occurred during 3-16 March. The ash plume on 7 March rose about 2 km above the crater and drifted SW. During an overflight in the middle of March, researchers from CUEIV-UdC noted degassing from small explosion craters on the floor of the main crater; there was no evidence of effusive activity or growth of a new dome. After the middle of March, seismicity steadily decreased; CUEIV-UdC reported landslides every week through June, but no additional ash emissions were reported.

The MIROVA radiative power plot of the MODIS thermal anomaly data clearly shows the thermal activity at Colima during September 2016-February 2017 (figure 133).

Figure 133. MIROVA log radiative power data from MODIS thermal anomaly satellite information clearly shows the strong thermal anomalies from the lava flows at Colima during September 2016-February 2017. The thermal anomalies shown in black after February 2017 are not located on the edifice and are not related to volcanic activity. Courtesy of MIROVA.

Geologic Background. The Colima complex is the most prominent volcanic center of the western Mexican Volcanic Belt. It consists of two southward-younging volcanoes, Nevado de Colima (the high point of the complex) on the north and the historically active Volcán de Colima at the south. A group of late-Pleistocene cinder cones is located on the floor of the Colima graben west and east of the complex. Volcán de Colima (also known as Volcán Fuego) is a youthful stratovolcano constructed within a 5-km-wide scarp, breached to the south, that has been the source of large debris avalanches. Major slope failures have occurred repeatedly from both the Nevado and Colima cones, producing thick debris-avalanche deposits on three sides of the complex. Frequent recorded eruptions date back to the 16th century. Occasional major explosive eruptions have destroyed the summit (most recently in 1913) and left a deep, steep-sided crater that was slowly refilled and then overtopped by lava dome growth.

Information Contacts: Unidad Estatal de Protección Civil de Colima (UEPCC), Roberto Esperón 1170 Col. de los Trabajadores, C.P. 28020 (URL: http://www.proteccioncivil.col.gob.mx/); Centro Universitario de Estudios e Investigaciones de Vulcanologia (CUEIV-UdC), Universidad de Colima, Colima, Col. 28045, México; Centro Universitario de Estudios Vulcanologicos y Facultad de Ciencias de la Universidad de Colima, Avenida Universidad 333, Colima, Col., 28045 México (URL: http://portal.ucol.mx/cueiv/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Goddard Space Flight Center (NASA/GSFC), Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Webcams de México (URL: http://www.webcamsdemexico.com/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/); SkyAlert, Twitter (@SkyAlertMx) (URL: https://twitter.com/SkyAlertMx/status/705188862318882816); Sergio Tapiro, Twitter (@tapirofoto); Noticieros Televisa (URL: http://noticeros.televisa.com).

Following explosions that produced ash plumes in early July 2010 (BGVN 36:07), no additional activity was noted from Ebeko by the Kamchatkan Volcanic Eruption Response Team (KVERT) until October 2016. This rather remote volcano on the N end of Paramushir Island in the Kuril Islands (figure 6) contains many craters, lakes, and thermal features (figure 7). Ash plumes were observed on 20 October 2016 and continued to be detected intermittently through 19 April 2017 (table 4).

Figure 6. Satellite imagery from Google Earth showing the location of Ebeko volcano on the N end of Paramushir Island, Kuril Islands. The village and seaport of Severo-Kurilsk, the largest populated center on the island, is about 7 km E. Courtesy of Google Earth; specific sources of data are shown on the image.

Figure 7. Sketch map showing features in the crater area of Ebeko volcano. (1) thermal fields in pink, (2) fumaroles, (3) pots of thermal water, (4) crater lakes in blue, (5) rims of major craters. Roman numerals denote thermal fields: (I) Active Funnel (in the North Crater), (II) South Crater, (III) West Field, (IV) Northeastern Field, (V) Gremuchaya fumarole field, (VI) Florenskii fumarole field, (VII) First Eastern Field, (VIII) Second Eastern Field, (IX) Southeastern Field, (X) Lagernyi Brook field, (XI) Second Southeastern Field, (XII) Third Southeastern Field. From Rychagov and others, 2010.

Table 4. Summary of activity at Ebeko volcano from mid-October 2016 to mid-April 2017. ACC is Aviation Color Code. Data courtesy of KVERT.

According to observers about 7 km E in the city of Severo-Kurilsk, a gas-and-steam plume containing a small amount of ash rose from Ebeko on 20 October 2016 (figure 8), marking the start of its most recent eruption. The Aviation Color Code (ACC) was raised from Green to Yellow. Later that day observers noted gas, steam, and ash plumes rising from the volcano. Ground-based and satellite observations during 21-23 October indicated quiet conditions; consequently, the ACC was lowered to Green on 24 October.

Figure 8. Ash explosions from Ebeko at 2245 UTC on 19 October 2016 were photographed from Severo-Kurilsk, 7 km E of the volcano. Photo by T. Kotenko; courtesy of Institute of Volcanology and Seismology FEB RAS, KVERT.

On 8-9 December 2016 the ACC was again raised to Yellow when a gas and steam plume containing a small amount of ash was observed. Ash rose from both Sredniy Crater (middle) and Severny Crater (northern) during 9-10 December (figure 9). Further ash plumes were seen during 17-27 December the ACC was raised to Orange. Minor ash was reported during 30 December 2016-6 January 2017, along with gas and steam plumes. An ash plume rose up to 2 km altitude on 19 January (figure 10), and ash fell in Severo-Kurilsk on 30 January. More frequent explosions took place between 24 February and 19 April 2017 (table 4). Simultaneous explosions from two craters was observed on 15 April (figure 11).

Figure 9. Explosive ash eruption from the Ebeko craters at 0116 UTC on 10 December 2016. Photo by L. Kotenko; courtesy of Institute of Volcanology and Seismology FEB RAS, KVERT.

Figure 10. Ash from an explosive eruption of Ebeko on 19 January 2017 rose up to 2 km altitude. Photo by T. Kotenko; courtesy of Institute of Volcanology and Seismology FEB RAS, KVERT.

Figure 11. Explosions at Ebeko generated ash plumes simultaneously from the active Severny (northern) and Sredniy (middle) craters on 15 April 2017. Photo by T. Kotenko; courtesy of Institute of Volcanology and Seismology FEB RAS, KVERT.

Satellite thermal data from MODVOLC showed no thermal alerts for at least the last 10 years, and MIROVA only identified two low-power anomalies in the past year, one in late February 2017 and the other in late March 2017.

Reference: Rychagov S.N., Belousov V.I., Kotenko ?.A., and Kotenko L.V., 2010, Gas-hydrothermal system of the geothermal deposit, Proceedings World Geothermal Congress 2010 Bali, Indonesia, 25-29 April 2010, 4 p.

Geologic Background. The flat-topped summit of the central cone of Ebeko volcano, one of the most active in the Kuril Islands, occupies the northern end of Paramushir Island. Three summit craters located along a SSW-NNE line form Ebeko volcano proper, at the northern end of a complex of five volcanic cones. Blocky lava flows extend west from Ebeko and SE from the neighboring Nezametnyi cone. The eastern part of the southern crater contains strong solfataras and a large boiling spring. The central crater is filled by a lake about 20 m deep whose shores are lined with steaming solfataras; the northern crater lies across a narrow, low barrier from the central crater and contains a small, cold crescentic lake. Historical activity, recorded since the late-18th century, has been restricted to small-to-moderate explosive eruptions from the summit craters. Intense fumarolic activity occurs in the summit craters, on the outer flanks of the cone, and in lateral explosion craters.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

Karymsky volcano on Russia's Kamchatka Peninsula has a lengthy eruptive history based on both radiocarbon data (back to about 6600 BCE) and historical observations (back to 1771). Much of the volcanic cone is surrounded by lava flows less than 200 years old. The most recent activity, consisting of frequent ash explosions and a few lava flows deposited on the flanks, has been ongoing for several decades. The most recent previous report described numerous ash explosions, persistent thermal anomalies, and moderate seismic activity through 2014 (BGVN 40:09). This report covers similar activity from January 2015 through May 2017. Information was compiled from the Kamchatka Volcanic Eruptions Response Team (KVERT), the Tokyo Volcanic Ash Advisory Center (VAAC), and several sources of satellite data.

Ash-bearing explosions and thermal anomalies characterized activity throughout 2015, beginning with an explosion on 19 January. Ash plumes were common through early March 2016, after which only steam-and-gas emissions and occasional thermal anomalies were noted, although fresh ash deposits were observed near the volcano in the second half of March. A brief episode of explosive activity during 5-8 October 2016 produced low-level ash plumes that drifted for hundreds of kilometers. No additional activity was reported through May 2017.

Activity during 2015. An explosive event at Karymsky on 19 January 2015 signaled a return to activity after a few months of quiet. The ash plume from the explosion extended 50 km SE, and the NASA Earth Observatory captured a satellite image showing trace ash deposits from the event trending SE across the snow-covered landscape (figure 34). Ashfall deposits were seen on 1 March (10-15 km E and SE) and 7 March.

Figure 34. A streak of dark ash extends SE from Karymsky's summit amidst a backdrop of snow on 18 January 2015 (UTC). The Operational Land Imager (OLI) aboard the Landsat 8 satellite acquired this natural color satellite image. Courtesy of NASA Earth Observatory.

Throughout the year, KVERT reported multiple thermal anomalies and ash plumes each month (table 8). The Tokyo VAAC issued 192 aviation alerts during the year, and the MODVOLC system reported eight thermal alerts in January, one in July, and two in August. Ash plume altitudes ranged from 2.1 to 7 km. Continuous ash emissions were noted during 16 and 29-30 July. The ash plume observed in satellite data on 17 July was 8 km long and 5 km wide. Volcanologists observed multiple explosions during 21-22 July, and helicopter pilots in the area reported explosions on 28 July that then lasted for several days (figure 35). Large plumes were also noted during December; on 22 December one was 8 km long and 6 km wide, and on 25 December one was 56 km long and 6 km wide. The highest altitude plumes were reported at 7 km drifting N on 16 and 20 November 2015 by the Tokyo VAAC. Ash plumes drifted in various directions, and were observed as far as 250 km before dissipating.

Table 8. Summary by month of ash plumes and thermal anomalies reported for Karymsky during 2015. Details include dates of thermal anomalies and ash plumes, maximum plume altitude in kilometers, distance in kilometers of ash plume drift, and direction of drift. Multiple thermal anomalies on a given date are shown in parentheses- 23(4)-after the date. 'Date: 7/8' means time zone boundaries presented different reported days for Kamchatka time (KST) and Universal Time (UTC). Sources are KVERT and Tokyo VAAC for ash plume data; KVERT and MODVOLC for thermal data.

Figure 35. Ash plume from an explosion at Karymsky on 30 July 2015. Photo by E. Kalacheva, IVS FEB RAS, courtesy of KVERT.

Activity during January 2016-April 2017. Activity was variable at Karymsky during 2016 (table 9). The Tokyo VAAC issued 132 aviation notices. Ash plumes and thermal anomalies were most frequent during January and February, with over twenty instances of each during February. The plume heights during February exceeded 6 km altitude four times, with the highest plume of the year on 20 February at 7.6 km altitude. Near-continuous ash emissions during the last week of February resulted in satellite observations of ash deposits around the volcano at the end of the month and during the first few days of March (figure 36). Activity decreased significantly during March, although KVERT noted fresh ash deposits again during 18-25 March. Except for thermal anomalies noted on 1 and 6 April, only steam-and-gas emissions were reported; KVERT lowered the Aviation Alert Level from Orange to Yellow (on a four-color scale) at the end of the month. From May to July, KVERT reported a thermal anomaly once each month. Steam-and-gas emissions were the only activity reported in August, and on 2 September, they lowered the Alert Level from Yellow to Green.

Table 9. Summary by month of ash plumes and thermal anomalies reported for Karymsky during 2016. Details include dates of thermal anomalies and ash plumes, maximum plume altitude in kilometers, distance in kilometers of ash plume drift, and direction of drift. Sources are KVERT and Tokyo VAAC for ash plume data; KVERT and MODVOLC for thermal data.

Figure 36. Steam plume from Karymsky on 21 February 2016, and abundant fresh ashfall around the volcano from recent ash emissions. Photo by E. Nenasheva, courtesy of KVERT.

After six months of quiet, the Tokyo VAAC reported an ash plume on 5 (UTC)/6 (KST) October at 2.4 km altitude extending SE. Aviation alerts were issued through 8 October 2016. Although residing at a fairly low altitude (2.4 km), the plume observed in satellite imagery during 7-8 October was visible in satellite imagery drifting 390 km E and SE before dissipating. KVERT briefly raised the Alert Level to Yellow and then to Orange on 7 and 8 October, and then back to Yellow on 19 October. Three weak thermal anomalies appeared in October and one in November; KVERT lowered the Alert Level to Green on 25 November. Karymsky remained at Alert Level Green through May 2017 with no further reports issued from KVERT or the Tokyo VAAC.

Geologic Background. Karymsky, the most active volcano of Kamchatka's eastern volcanic zone, is a symmetrical stratovolcano constructed within a 5-km-wide caldera that formed during the early Holocene. The caldera cuts the south side of the Pleistocene Dvor volcano and is located outside the north margin of the large mid-Pleistocene Polovinka caldera, which contains the smaller Akademia Nauk and Odnoboky calderas. Most seismicity preceding Karymsky eruptions originated beneath Akademia Nauk caldera, located immediately south. The caldera enclosing Karymsky formed about 7600-7700 radiocarbon years ago; construction of the stratovolcano began about 2000 years later. The latest eruptive period began about 500 years ago, following a 2300-year quiescence. Much of the cone is mantled by lava flows less than 200 years old. Historical eruptions have been vulcanian or vulcanian-strombolian with moderate explosive activity and occasional lava flows from the summit crater.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far Eastern Branch, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences, (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/).

Hawaii's Kīlauea volcano continues the long-term eruptive activity that began in 1983 with lava flows from the East Rift Zone (ERZ) and a convecting lava lake inside Halema'uma'u crater. The US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO) has been monitoring and researching the volcano for over a century since its founding in 1912. HVO provided quarterly reports of activity for July-December 2016, which are summarized below.

Summary of July-December 2016 activity. Activity at Kīlauea during the second half of 2016 was consistent with long-term trends of summit inflation punctuated by DI (Deflation-Inflation) events and a slowly rising average lava lake level inside Halema?uma?u crater. Two explosive events prompted by rockfalls into the lake sent spatter high enough to reach the Halema?uma?u rim; a small overflow at the crater occurred in October, the first since April-May 2015.

Pu'u 'O'o activity continued with little change except for the steady advance of the episode 61g lava flow towards the coast. The pahoehoe front reached the Emergency Access Road near the coast on 25 July and cascaded slowly over the seacliff into the ocean on 26 July just after midnight. It was the first time since August 2013 that lava from Pu'u 'O'o entered the sea. A growing lava delta of about 10 hectares (25 acres) at the Kamokuna entry was the focus of attention by visitors until most of it collapsed into the sea on 31 December 2016.

Activity at Halema'uma'u. Eruptive activity at Halema'uma'u crater was typical during July-December 2016, with a slightly elevated lake level for the last quarter of the year. The lava lake circulation pattern continued in the usual N-S direction, with occasional shifts due to short-lived spattering in areas other than the normal Southeast sink. The lake level rose and fell in concert with the regular summit DI events. On 7 September, the lake level rose to the level of the old rim prior to the April/May 2015 crater overflow, 8 m below the current rim (figure 267).

Figure 267. Halema'uma'u lava lake at Kīlauea on 7 September 2016 at 1842 HST when the surface level was at the level of the old crater rim, 8 m below the current rim. Photo by M. Patrick, courtesy of Hawaiian Volcano Observatory (HVO) (Hawaiian Volcano Observatory Quarterly Report for July-September 2016).

The lowest lake level of the period was 55 m below the floor of Halema'uma'u on 6 October; the lake reached its highest level when it overflowed the rim on 15 October. This was the highest lake level since the overflows in late April to early May 2015, and it covered a small area of about 5,000 m² of the Halema'uma'u crater floor. The latest overflows consisted of two small lobes that spilled onto the crater floor on the SE and NW sides of the lake (figure 268).

Figure 268. Aerial photo of Halema'uma'u crater and lava lake at Kīlauea, looking south, showing the two areas where the lake overflowed onto the crater floor on 15 October 2016. The first overflow is on the upper-left side of the lake; the later overflow at is at the lower right side. Photo by T. Orr, 3 November 2016, courtesy of HVO (Hawaiian Volcano Observatory Quarterly Report for October-December 2016).

The lava lake surface or spatter from the lake was visible intermittently from the Jaggar Overlook on the NW rim of the caldera. During a few of the deflation phases of the DI events, newly exposed juvenile veneer on the crater walls detached and collapsed into the lake. Many of these collapses were too small to notice on the webcams or produce seismic events, but several events were noteworthy.

On 6 August there was a large collapse at the base of the Halema'uma'u crater wall (above the Southeast sink). The collapse produced a large explosive event, along with a composite seismic event, and vigorous spattering. The main explosive deposit blanketed the rim just east of the closed overlook, with tephra forming a continuous layer up to 20 cm thick. Bombs were deposited over an area 220 m wide (along the rim) and up to 90 m beyond the crater rim, with sparse lapilli thrown across the parking lot. HVO monitoring equipment and some of the remaining wooden fencing for the overlook were burned.

A second explosive event occurred on 19 September, also triggered by a collapse of the crater wall above the Southeast sink. Bombs and smaller scoria reached the Halema'uma'u crater rim and ash was deposited across the parking area and road. Large events also occurred on 4 October around 1100, and again at around noon. The first triggered a composite seismic event and spattering when veneer on the E wall fell into the lake, and the other triggered brief spattering when a large sheet of veneer fell from the SW wall. On 19 and 20 October, explosive events were triggered by rockfalls below the overlook (figure 269). The first, at 0745, deposited spatter and ribbon bombs up to 30 cm long on the rim of Halema'uma'u, and produced muted composite seismicity. The 20 October event occurred at 1225, producing a tephra deposit that extended across the road past the parking lot, and generated weak composite seismicity.

Figure 269. Bombs and spatter from Halema'uma'u crater at Kīlauea during October and November 2016. Left: the 20 October explosive event from the HMcam (a webcam on the SE rim of the crater) taken at 1226, showing spatter bombarding the overlook, after the collapse of the crater wall below the webcam. Right: a large bomb thrown from the lava lake during the 28 November explosive event. The fluidity of the spatter allowed it to splat upon impact. Photo by M. Patrick, 28 November 2016, courtesy of HVO (Hawaiian Volcano Observatory Quarterly Report for October-December 2016).

At 1159 on 28 November, another slice of crater wall below the HMcam (one of two Halema'uma'u webcams) fell and triggered an explosive event that again threw tephra onto the rim. The tephra deposit was sparse and confined to a narrow area 90-100 m wide along the rim between the two webcams on the SE rim. While most of the spatter bombs were less than 30 cm in size, the largest was about 160 cm long. The clasts were relatively fluidal in texture and most splatted upon impact (figure 269). The power and Ethernet cables for one of the webcams were damaged during this event. A similar event occurred on 2 December at 0658, when a large slab from the overlook crater wall directly below one of the webcams collapsed. This also triggered a small explosive event which bombarded the rim with spatter near the two cameras, and produced rare ribbon bombs close to a meter long. Another large veneer collapse occurred on 13 December at about 1355, when a slab fell from the N wall into the lake and triggered spattering.

Activity at Pu'u 'O'o and the East Rift Zone. There were few notable changes at Pu'u 'O'o cone from July through December. Very slight uplift was observed during 2-4 July that may have corresponded to inflationary tilt. The forked lava stream in the vent on the NE spillway was visible on a 15 July overflight. Subsequent overflights found the streams progressively more crusted over, and no lava was visible in the vent on the 19 August overflight. The W pit had a large collapse of its NE rim that was noticed on 1 September. A few meters had shaved off the rim of the pit, making a pile of rubble on the pit floor.

One of the two vents on the NE spillway re-opened at some point during the day on 2 November. Fieldwork on 3 November showed that the W-pit lava pond was 52 m across and 22 m below the pit rim, at an elevation of 848 m. The pond level was at 847 m when seen again on 29 November, with weak spattering at a few places around the pond perimeter.

The new flow (episode 61g), which began from the NE flank of Pu'u 'O'o cone on 24 May 2016, had reached the top of Pulama pali (cliff) on 28 June 2016 (BGVN 41:08, figure 263). It reached the base of the pali on the last day of June, and began to advance quickly across the coastal plain (figure 270). It was initially quite narrow, about 100 m across, possibly because of the flow high advance rate and confining topography in the area, according to HVO. The flow had slowed by 5 July; it was half way across the coastal plain, with the leading tip about 1.7 km from both the base of the pali and the ocean, and 1.6 km from the closest portion of the FEMA evacuation road that runs along the coast.

Figure 270. Episode 61g lava flow at Kīlauea leaves the base of Pulama pali headed across the coastal plain on 2 July 2016. Several channelized 'a'a flows are visible coming down the slope. Location is at the eastern boundary of the National Park and western boundary of the Royal Gardens subdivision. Photo by Kirsten Stephens, courtesy of Hawaiian Volcano Observatory (HVO) (Hawaiian Volcano Observatory Quarterly Report for July-September 2016).

The flow front continued to advance slowly over the next few weeks and eventually stalled in mid-July. The stalled front was soon overtaken, however, by breakouts that had been steadily advancing downslope behind the front. These breakouts formed a new front that continued to advance rapidly at up to 170 m/day. By 24 July, the flow front had reached to within about 260 m of the FEMA emergency access road. The next day (25 July) at 1520 HST, the 61g flow crossed the FEMA road (figure 271), and at 0112 HST on 26 July lava spilled over the sea cliff and into the water, marking the start of the rapid growth of the Kamokuna ocean entry.

Figure 271. Episode 61g lava flow of Kīlauea crosses the FEMA emergency access road. Left: the lava flow on 25 July 2016 at 1616 HST about 30 minutes after it crossed the road in a thin sheet, photo by L. DeSmither. Right: on 5 August (almost two weeks later), in the same general location as the first, note the amount of flow inflation (HVO geologist for scale), photo by M. Patrick. Both images courtesy of Hawaiian Volcano Observatory (HVO) (Hawaiian Volcano Observatory Quarterly Report for July-September 2016).

The flow field continued to widen over the next few months, as scattered breakouts crept down the flow (figure 272). One of these breakouts formed a second ocean entry point several hundred meters to the W of the initial entry. Other, smaller breakouts reached the ocean along the stretch of land between the two main entry points, forming short-lived entries (figure 273). Persistent breakouts near the base of the Pulama pali began to build a ramp, making the pali less steep.

Figure 272. A breakout from the episode 61g flow on the coastal plain of Kīlauea on 20 September 2016. Burning vegetation on the pali from the recent flow is visible in the background. Photo by Matt Patrick, courtesy of Hawaiian Volcano Observatory (HVO) (Hawaiian Volcano Observatory Quarterly Report for July-September 2016).

Figure 273. Lava flows into the sea at Kīlauea from one of the entry points along the Kamokuna ocean entry, as viewed from the sea, on 11 September 2016. Photo by Tom Pfeiffer, courtesy of Volcano Discovery.

Numerous small delta collapses on both the E and W deltas were reported during August and September, but the deltas overall continued to grow. By the end of September the E delta was about 5.2 hectares (12.9 acres), and had developed several large coast-parallel cracks that suggested it was becoming unstable (figure 274). Activity at the W delta was always subordinate to that at the E delta and was abandoned in late September, having reached about 2.6 ha in size.

Figure 274. The E lava delta at the Kamokuna ocean entry at Kīlauea on 30 September 2016. Top: the E Kamokuna ocean entry and lava delta, showing large cracks parallel to the sea cliff. Photo by T. Orr. Bottom: thermal image of the delta showing heat in the cracks, and hot water plumes extending out from the ocean entry points. Courtesy of Hawaiian Volcano Observatory (HVO) (Hawaiian Volcano Observatory Quarterly Report for July-September 2016).

The only surface activity not on the lower half of the flow field (from the top of the pali to the coast) during July-September was a large breakout from the episode 61g vent on the east flank of Pu'u 'O'o cone that started 29 August. The breakout was active for only a few days and died during the first week of September. On 27 September a skylight abruptly opened a few hundred meters inland from the ocean entry, producing a strong glow at night. Very little surface activity was present on the coastal plain near the Kamokuna ocean entry during October-December. A small breakout started about a kilometer upslope from the park rope line on 24 November, and remained active until the evening of 28 November.

However, breakouts did continue near the Pulama pali during October-December, further building up the intermediate-sloped ramp at the base of the pali (figure 275). The first of these started on 1 October and continued until at least 23 October, having extended a short distance beyond the base of the pali. A breakout started near the bottom of the steepest part of the pali during 22-23 November, producing short-lived channelized flows. The breakout remained active until at least 30 November, but was apparently inactive by 6 December.

Figure 275. Episode 61g eruption of Kīlauea on 13 November 2016, captured by the Advanced Land Imager (ALI) on NASA's Earth Observing-1 satellite. The lava first reached the ocean on 26 July, and most of the lava delta created at the Kamokuna entry collapsed into the sea on 31 December 2016. The gray areas in the image show lava that has accumulated since 1983. The 2016 active flow started at a vent just east of the Pu'u 'O'o crater. It moved SE and S through lava tubes below the surface. The signature of a recent surface breakout is the lighter gray area at the base of the Pulama pali (cliff). Courtesy of NASA Earth Observatory.

At the episode 61g vent near Pu'u 'O'o cone, a new breakout started between 0830 and 0840 on 21 November 2016. The ground surface over and just upslope from the vent was fractured and uplifted 3-4 m. The breakout consisted of two branches, one of which generally headed S and was short lived, stagnating during the day of 26 November. The other flowed NE and surrounded the nearby Pu?u Halulu cone before turning to the SE. The flow front of this second branch was about 2 km from the vent when mapped on 17 December (figure 276), but continued to advance through the end of the year. In addition to the 21 November breakout, other short-lived breakouts from the episode 61g vent were active during 1-3 December, 11-12 December, and 25-28 December.

Figure 276. Changes to the flow field of the episode 61g flow between 20 September and 25 December 2016. Courtesy of HVO (Hawaiian Volcano Observatory Quarterly Report for October-December 2016).

During an overflight on 3 November, HVO found that the W delta, which became inactive in late September, was approximately 2 ha after losing about 0.6 ha to wave erosion. The E delta at the Kamokuna ocean entry remained very active through December, reaching a relatively stable size of around 10 ha, kept in check by frequent small collapses. Large cracks on the delta parallel to the old sea cliff were apparent, and the delta on the seaward side of the cracks appeared to be tilted, indicating instability. The delta was about 9 ha in size in late December.

During mid-afternoon on 31 December 2016, the E delta began to collapse in pieces. Over the course of a few hours, most of the delta had disappeared into the water, leaving about 1 ha as narrow remnant ledges at the base of the sea cliff (figures 277 and 278). In addition to the delta collapse, roughly 1.6 ha of the older, post-1986 sea cliff also fell into the ocean, likely due to undercutting promoted by the delta collapse. This portion of the old sea cliff was partially above the E edge of the delta, but most of it was adjacent to the delta to the east (figure 278), and included part of the National Park viewing area. The sea cliff collapses produced thick, dusty plumes and large waves that splashed back onto the sea cliff, in some instances. In the days that followed, a few more small slices of unstable sea cliff collapsed into the water. The total area that collapsed, including the delta and the older sea cliff, was approximately 10 ha.

Figure 277. Eastern Kamokuna lava delta (episode 61g flow) at Kīlauea, before and after the 31 December 2016 collapse. Left: The delta on 14 October when it was about 6 ha (15 acres) in size. Photo by L. DeSmither. Right: After the 31 December collapse, showing remnants of the delta. Photo by M. Patrick on 1 January 2017. Courtesy of HVO (Hawaiian Volcano Observatory Quarterly Report for October-December 2016).

Figure 278. Map of the Kamokuna ocean entry at Kīlauea as of 3 January 2017, showing areas of collapse, remaining delta, and other features. Courtesy of HVO (Hawaiian Volcano Observatory Quarterly Report for October-December 2016).

Thermal anomaly data. Satellite-based thermal anomaly data from the MODIS instrument generates a strong continuous signal from Kīlauea that closely follows the distribution of the active lava flows. As the episode 61g flow emerged from Pu'u 'O'o and headed SE, the thermal signature was strong between Pu'u 'O'o and the Pulama pali during the last week of June as recorded by the University of Hawaii's MODVOLC thermal alert system. By mid-August, a few weeks after the flow had reached the sea, the thermal activity extended from the pali to the Kamokuna ocean entry site (figure 279).

Figure 279. Thermal alerts from MODVOLC at Kīlauea during late June and August 2016. Pu'u 'O'o is beneath the pixel in the upper left of the top image. Top: Alerts during 26 June-1 July 2016. The Pulama pali shows as the shaded area underneath the leading SE edge of the flow. Bottom: Alerts during 12-19 August 2016. The lava was hottest between the Pulama pali on the N and the new Kamokuna ocean entry at the bottom of the image. Courtesy of HIGP MODVOLC Thermal Alerts System.

New breakouts from the Pulama pali area were recorded as thermal alerts during the second week of November along with the evidence for continued thermal alerts from the Kamokuna delta at the shoreline. At the vent area of episode 61g, near Pu'u 'O'o cone, new breakouts flowed NE of the cone and were captured as thermal alerts during early December (figure 280).

Figure 280. Thermal alerts from MODVOLC at Kīlauea during November and December 2016. Top: New breakouts were reported from the Pulama pali area and were visible in the thermal data during 5-11 November along with the thermal alerts from the Kamokuna lava delta at the shoreline. Bottom: Alerts during 10-16 December 2016 show renewed breakout activity at the episode 61g vent near Pu'u 'O'o (upper left of image) as well as continued activity at the Kamokuna ocean entry on the shoreline. Courtesy of HIGP MODVOLC Thermal Alerts System.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: https://volcanoes.usgs.gov/observatories/hvo/); NASA Earth Observatory, EOS Project Science Office, NASA Goddard Space Flight Center, Goddard, Maryland, USA (URL: http://earthobservatory.nasa.gov/); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Tom Pfeiffer, Volcano Discovery (URL: http://www.volcanodiscovery.com/).

The active crater at Costa Rica's Rincón de la Vieja, which contains a 500-m-wide acid lake, has been the site of numerous historic eruptions at this large volcanic complex. Intermittent phreatic explosions since 2011 have dispersed volcanic debris from the crater lake within a few kilometers of the crater rim and into the surrounding streams a number of times. The most recent previous activity included explosions in September and October 2014, and phreatic eruptions on June, August and October 2015 (BGVN 41:01); this report discusses activity during 2016 and through July 2017. Information comes from the Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA) and the Observatorio Sismológico y Vulcanológico de Arenal-Miravalles (OSIVAM-ICE). The OVISAM-ICE reports are published through the Red Sismológica Nacional (RSN), the National Seismological Network. Ejected material is described in the original reports in various ways that appear to be interchangeable rather than signifying actual content differences, so those distinctions are not reflected below unless ash was specified.

The first evidence of a new episode of phreatic explosions was noted during a site visit on 15 February 2016. Numerous explosions during March spread material as far as 2 km from the crater rim. After an explosion on 1 May 2016 there were no further reports until 23 May 2017, when a series of intermittent explosions again ejected material onto the N and NW flanks and sent plumes of steam-and-gas as high as 2 km above the crater rim. The last reported explosion was on 5 July 2017.

Activity decreased at the end of 2015 after the phreatic explosions of 16-21 October. The number of seismic events increased again during February and March 2016. OVSICORI-UNA scientists observed the first evidence of a new episode of phreatic explosions during a field visit on 15 February 2016 when they noted deposits about 20 m from the crater rim. By the end of March, the RSN had reported 25 explosions. Three of the largest explosions occurred on 9 February, 9 March, and 18 March. They were characterized by episodes of tremor in pulses that usually lasted about five minutes prior to the phreatic explosion, and then changed to continuous tremor for several hours afterwards.

OSIVAM-ICE scientists reported photographic evidence of deposits from a 2 March explosion that covered a wide area on the N flank of the active crater (figure 22). They visited on 3 March 2016 and noted fresh deposits from the phreatic explosions about 200 m W of the crater rim (figure 23). They also witnessed three explosions during the afternoon, the longest lasting for 65 seconds.

Figure 22. Deposits of material ejected from the crater lake on the N edge of Rincón de la Vieja associated with an eruptive event that occurred on 2 March 2016 at 1747 local time. Photo from Fernando Madrigal's Sensoria site, courtesy of RSN (Resumen de la actividad sísmica y eruptive del volcán Rincón de la Vieja (Costa Rica) 01 de octubre del 2015 al 15 de marzo del 2016).

Figure 23. Deposit of material from the crater lake at Rincón de la Vieja on 3 March 2016, located about 200 m W of the crater rim. Photo by OSIVAM-ICE scientists, courtesy of RSN (Resumen de la actividad sísmica y eruptive del volcán Rincón de la Vieja (Costa Rica) 01 de octubre del 2015 al 15 de marzo del 2016).

Scientists from OVSICORI-UNA conducted additional site visits during 8 and 10-11 March 2016. On 8 March fresh ash was found about 120 m from the crater rim (figure 24), and a temperature of 55°C was measured remotely for the convection cell in the lake. Based on photographs taken by nearby residents, OVSICORI-UNA scientists estimated that the ash and steam plumes produced by the 9 and 10 March explosions rose 700 and 850 m, respectively, above the crater. Local residents reported to The Tico Times that ash fell on the roofs of their homes within an area up to 6 km around the volcano after the explosion on 9 March, mostly in communities N of the crater (Upala and Buenos Aires).

Figure 24. The N rim of the active crater at Rincón de la Vieja on 8 March 2016 is marked with the outline (white dashes) showing the extent of material ejected during recent explosions. The arrow at the top shows the dominant wind direction. Inset on left shows riverbed deposits of recent material on 8 March, and the right inset images show the plumes from the 9 (upper) and 10 (lower) March explosions. Right inset photos by Jorge Viales, courtesy of OVSICORI (Erupciones del volcán Rincón de la Vieja: Observaciones de Campo).

The character of the deposits changed between February and March 2016, according to a report by OVSICORI scientists. The samples collected in February were rich in elemental sulfur, abundant in the crater lake and in the near-surface sediments. Studies of the March samples showed the presence of clasts of altered rocks, hydrothermal minerals, and elemental sulfur as well as 3-10% fresh glass.

During their summit visit on 10 and 11 March 2016, OVSICORI scientists noted a coating of white sediment, up to 5 mm thick in some places, covering the ground and the vegetation in a 400m-wide area to the SSW of the active crater (figure 25). Deposits extended as far as 2 km away, and coated the flanks of both the active crater and the nearby Von Seeback crater (figure 26).

Figure 25. Material from phreatic explosions cover a Copey shrub at Rincón de la Vieja on 10 March 2016. The plant was located 1.5 km SSW from the active crater. Photo by E. Duarte, courtesy of OVSICORI-UNA (Visita al Volcán Rincón de la Vieja: Mapeo de Efecto y Características de Erupciones Freáticas Recientes).

Figure 26. A view to the ESE on 10 March 2016 from the flank of the Von Seeback crater towards the active crater showing the coating of white sediments from the recent phreatic explosions at Rincón de la Vieja. The arrow points roughly NW showing the direction of sediment dispersal. Material was sampled at site 4 (white circle). Photo by E. Duarte, courtesy of OVSICORI-UNA (Visita al Volcán Rincón de la Vieja: Mapeo de Efecto y Características de Erupciones Freáticas Recientes).

A 15 March explosion generated a 700-m-high plume of water vapor and gas, according to an announcement from OVSICORI-UNA. They also reported an explosion on 1 May 2016 detected for 11 minutes by the seismic network. No further reports were made until May 2017.

A small lahar traveled down the N flank of the crater after an explosion on 23 May 2017. Explosions on 11 and 12 June were recorded seismically, but cloudy weather obscured visual observations. The Washington VAAC, however, noted a hotspot in the infrared satellite data on 11 June 2017 about 30 minutes before the explosion was reported. A diffuse steam plume was observed from Dos Rios de Upala rising about 50 m above the summit on 15 June, and a small phreatic explosion was recorded on 18 June 2017. A larger explosion on 23 June sent a plume 1-2 km above the summit, and ejected material to the W and NW onto the upper N flank toward the Von Seebach crater 2 km to the W. Small phreatic explosions on 5 July ejected material that did not rise above the crater rim.

Geologic Background. Rincón de la Vieja, the largest volcano in NW Costa Rica, is a remote volcanic complex in the Guanacaste Range. The volcano consists of an elongated, arcuate NW-SE-trending ridge constructed within the 15-km-wide early Pleistocene Guachipelín caldera, whose rim is exposed on the south side. Sometimes known as the "Colossus of Guanacaste," it has an estimated volume of 130 km3 and contains at least nine major eruptive centers. Activity has migrated to the SE, where the youngest-looking craters are located. The twin cone of Santa María volcano, the highest peak of the complex, is located at the eastern end of a smaller, 5-km-wide caldera and has a 500-m-wide crater. A Plinian eruption producing the 0.25 km3 Río Blanca tephra about 3,500 years ago was the last major magmatic eruption. All subsequent eruptions, including numerous historical eruptions possibly dating back to the 16th century, have been from the prominent active crater containing a 500-m-wide acid lake located ENE of Von Seebach crater.

Information Contacts: Observatorio Vulcanológico Sismológica de Costa Rica-Universidad Nacional (OVSICORI-UNA), Apartado 86-3000, Heredia, Costa Rica (URL: http://www.ovsicori.una.ac.cr/); Observatorio Sismológico y Vulcanológico Arenal-Miravalles del Instituto Costarricense de Electricidad (OSIVAM-ICE), Sección de Sismología, Vulcanología y Exploración Geofísica, Escuela Centroamericana de Geología, Apdo. 214-2060, San Pedro, Costa Rica (URL: http://rsn.ucr.ac.cr/); The Tico Times (URL: http://www.ticotimes.net/2016/03/10/costa-rica-rincon-de-la-vieja-volcano-vapor-ash-explosions).

Ecuador's Sangay, isolated on the east side of the Andean crest, has exhibited frequent eruptive activity over the last 400 years. Its remoteness has made ground observations difficult until recent times, and thus most information has come from aviation reports from the Washington Volcanic Ash Advisory Center (VAAC) and MODIS (Moderate Resolution Imaging Spectroradiometer) satellite-based data. Thermal anomaly information is reported by the University of Hawaii's MODVOLC system and the Italian MIROVA Volcano HotSpot Detection System. Ecuador's Instituto Geofísico (IG) issues periodic Special Reports of activity. This report summarizes the intermittent nature of the eruptions from 2011-2013, and covers renewed activity during January 2015 through July 2017.

Summary of activity during 2011-2013. Activity during 2011 (figure 17) began with a continuation of the intermittent ash emissions and thermal anomalies that persisted throughout 2010 (BGVN 36:01). Ash plumes during January and February 2011 were reported at typical altitudes between 6 and 8 km; thermal alerts appeared once each during January and March. No activity was reported after 2 March until a new series of thermal alerts began more than 3 months later on 6 June 2011; they were intermittent from then through 19 September 2012, with reports occurring during 1-4 days of all but three months. Ash emissions were also intermittent during this time, with VAAC reports issued during eight of the months from 2 August 2011-28 July 2012 for plumes reported at altitudes of 6-8 km. They also generally occurred during 1-4 days of the month. A four-month break in activity followed until ash plumes were reported on 25 January 2013; they were intermittent until 24 May 2013. MODVOLC thermal anomalies were also reported during this time, on 2 February, 25 March, and 3-4 May.

Figure 17. Summary chart of ash emissions and thermal anomalies reported from Sangay during January 2010 to early August 2017. Red bars show eruptive periods where there are reports of either ash plumes or thermal anomalies without a lack of observed activity for more than 3 months. Rows with pink cells indicate dates with thermal anomalies (MODVOLC or MIROVA). Rows with blue cells indicate dates with ash emissions as reported by the Washington VAAC. A range of dates means that activity occurred at least on those two dates, but may not have been continuous. Data courtesy of Washington VAAC, HIGP MODVOLC Thermal Alerts System, and MIROVA.

Summary of activity during January 2015-July 2017. After 19 months of quiet from June 2013 through December 2014, an ash plume reported on 19 January 2015 marked the beginning of a new eruptive episode that included ash plumes, lava flows, and block avalanches between 19 January and 7 April 2015. The next reported activity included both ash emissions and thermal anomalies observed almost a year later on 25 March 2016, although IG had reported increases in seismicity during the previous two weeks. Ash emissions and thermal anomalies were intermittent through 16 July 2016. There was a single thermal anomaly seen in MIROVA data on about 10 October and a brief ash emission occurred during 16-17 November 2016, after which Sangay was quiet until a new episode started on 20 July 2017 that was ongoing into August.

Activity during January-April 2015. After a 19-month period of no reported activity (since May 2013), ash emissions were again seen beginning on 18 January 2015 when an ash plume rose to 6.4 km altitude and drifted SW. Additional plumes on 25 January and 4 February rose to 7.3 km and 6.7 km, respectively, and drifted less than 20 km SW (figure 18). Ash plumes primarily observed by pilots between 27 February and 16 March were generally not visible in satellite images due to weather clouds. During this episode, MODVOLC thermal alerts were reported on 26 January; 7, 21, 23 and 27 February; 2,4,18, and 27 March; and 1, 3, and 7 April.

Figure 18. Ash emission at Sangay sometime during 19-26 January 2015. The ash plume eventually reached about 2 km above the 5,286-m-high summit crater. Photo by Gustavo Cruz, courtesy of IG (Informe Especial del Volcan Sangay No 1, 16 March 2015).

In a March 2015 report, IG noted that new lava flows and block-avalanche deposits had been emplaced during January and February 2015. The lava flows descended the SE flank about 900 m (figure 19). Two areas of deposits from block avalanches and ashfall extended 2.5 km ESE from the lava front, and 1.5 km down the S flank. According to IG, there were 21 thermal anomalies identified in MIROVA during 31 January-25 February 2015.

Figure 19. Locations of lava flows and block-avalanche deposits at Sangay that were emplaced during January and February 2015. The new lava flows are shown in red. The ash and block-avalanche deposits are shown in stippled yellow/green. Courtesy of IG (Informe Especial del Volcan Sangay No 1, 16 March 2015).

Activity during March-November 2016. IG reported an increase in seismicity on 5 March 2016, after ten months of no reported activity. An explosion signal was followed by harmonic tremor on 9 March, and IG noted that both a thermal anomaly and an emission drifting S were identified in NOAA satellite images. They inferred that increased seismic "explosion" signals on 14 March were indicative of ash-and-gas emissions, although weather clouds prohibited visual confirmation. Ash emissions rising to 6.1 km altitude were first reported by the Guayaquil MWO on 25 March 2016; they noted two more emissions on 27 and 28 March rising to similar altitudes (7.6 and 6.4 km, respectively), but cloudy weather prevented satellite confirmation. Plumes reported on nine days during April rose to similar altitudes (ranging from 5.5-7 km) and extended 18-30 km N or NW from the summit. A series of daily emissions occurred from 30 April-7 May. The emissions included a plume on 2 May that extended 120 km NW, and one on 6 May that rose to 8.2 km altitude and extended approximately 55 km SW before dissipating. Ash-bearing plumes were reported on 10 more days during the rest of May.

Although no more ash plumes were reported until 16 July 2016, MODVOLC thermal alerts were persistent every month beginning on 25 March and lasting through 5 July (see figure 17 above). The MIROVA data for this period also clearly show persistent thermal anomalies (figure 20). A short-lived eruption event during 16-17 November 2016 consisted of an ash emission that rose to 6.1 km altitude and drifted as far as 290 km SE.

Figure 20. Thermal anomaly data from MIROVA for the year ending on 18 January 2017 at Sangay, showing the eruptive episode of March-July 2016, and a brief anomaly on about 10 October 2016; late October-November anomalies are more than 20 kilometers from the summit and unrelated to volcanism. Courtesy of MIROVA.

Activity beginning July 2017. A new eruptive episode began on 20 July 2017, after eight months without major surface activity. Low-energy ash emissions rising to 3 km above the crater, incandescent block avalanches on the ESE flank (figure 21), and a possible new lava flow were reported by IG. The Washington VAAC reported an ash emission on 20 July rising to 8.2 km altitude and drifting about 80 km W. A plume was reported on 1 August by the Guyaquil MWO but obscured by clouds in satellite images, and a plume on 2 August was seen in webcam images (figure 22).

Figure 21. Incandescent blocks roll down the ESE flank of Sangay during the early morning of 1 August 2017. Courtesy of IG (Informe Especial del Volcán Sangay-2017-No 1, 3 August 2017).

Figure 22. Ash emission at Sangay on 2 August 2017, with the plume rising about 400 m above the summit crater drifting SW. Courtesy of IG (Informe Especial del Volcán Sangay-2017-No 1, 3 August 2017).

Geologic Background. The isolated Sangay volcano, located east of the Andean crest, is the southernmost of Ecuador's volcanoes and its most active. The steep-sided, glacier-covered, dominantly andesitic volcano grew within the open calderas of two previous edifices which were destroyed by collapse to the east, producing large debris avalanches that reached the Amazonian lowlands. The modern edifice dates back to at least 14,000 years ago. It towers above the tropical jungle on the east side; on the other sides flat plains of ash have been eroded by heavy rains into steep-walled canyons up to 600 m deep. The earliest report of an eruption was in 1628. Almost continuous eruptions were reported from 1728 until 1916, and again from 1934 to the present. The almost constant activity has caused frequent changes to the morphology of the summit crater complex.

Information Contacts: Instituto Geofísico (IG), Escuela Politécnica Nacional, Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/, archive at: http://www.ssd.noaa.gov/VAAC/archive.html); Hawai'i Institute of Geophysics and Planetology (HIGP), MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

An eruption at Sheveluch has been ongoing since 1999, and recent activity there was previously described through February 2016 (BGVN 42:03). During March 2016-July 2017, the same type of activity prevailed as before, consisting of lava dome growth, explosions, and pyroclastic flows. The following data comes from Kamchatka Volcanic Eruption Response Team (KVERT) reports. During this period the Aviation Color Code (ACC) remained at Orange (the second highest level on a four-color scale), except for a brief period on 10 December 2016 and brief periods during May-July 2017 when it was Red (highest level).

Activity during March 2016-April 2017. According to KVERT, ongoing activity during March 2016-April 2017 consisted of lava-dome extrusion onto the N flank accompanied by strong fumarolic activity, dome incandescence, ash explosions, and hot avalanches. Satellite images detected an intense daily thermal anomaly over the dome.

On 18 September 2016, a moderate explosion caused dome collapse and 10-km-long pyroclastic flows. Pyroclastic-flow deposits were noted in the Baydarnaya (also spelled Baidarnaya) River valley to the SSW and in the central part of the S flank.

Ash plumes generated by explosions and re-suspended ash usually occurred several times per month, and generally reached altitudes of 4.5-7 km. On 10 December 2016, explosions generated ash plumes observed in satellite images that rose to altitudes of 10-11 km and drifted 910 km NNE. The ACC was raised to Red. By the following day, no further ash emissions were observed, and the ACC was lowered back to Orange. However, explosions continued in December that sent ash plumes as high as 7 km altitude (figure 42). Typical activity continued through the first few months of 2017, including ash explosions sending plumes to as high as 5-6 km altitude (figure 43) that remained visible in satellite imagery 100 km downwind.

Figure 42. Explosions from Sheveluch sent ash up to 7 km altitude at 2314 UTC on 19 December 2016. Photo by Yu. Demyanchuk; courtesy of Institute of Volcanology and Seismology FEB RAS, KVERT.

Figure 43. Typical activity from Sheveluch is evident on 16 April 2017, with an ash plume rising to around 4 km altitude. Photo by Yu. Demyanchuk; courtesy of Institute of Volcanology and Seismology FEB RAS, KVERT.

Activity during May-July 2017. Beginning In May 2017, explosive activity appeared to intensify. Strong explosions on 12 May 2017 generated ash plumes identified in satellite images that rose to altitudes of 9-10 km, spread 70 km wide, and drifted 115 km NW. The ACC was raised to Red. Pyroclastic flows descended the flanks and produced ash plumes that rose 3.5-4 km and drifted NE. A few hours later, satellite images showed a thermal anomaly but no ash emissions, and the ACC was lowered back to Orange.

According to KVERT, after a series of explosions during 13-14 May (figure 44), powerful explosions on 16 May generated ash plumes that rose 8-11 km in altitude, prompting an increase of the ACC to Red. Pyroclastic flows descended the S flank, producing ash plumes that rose 3.5-4 km in altitude (figure 43) and drifted NE; within a few hours, satellite images did not show any ash emissions; the ACC was lowered to Orange.

Figure 44. Ash plumes rise from explosive activity and pyroclastic flows at Sheveluch on 14 May 2017, seen here to an altitude of about 5 km. Photo by Yu. Demyanchuk; courtesy of Institute of Volcanology and Seismology FEB RAS, KVERT.

Additional explosions occurred 18 May. During 23-25 May 2017 powerful explosions generated ash plumes that rose to an altitude of 8 km and drifted 715 km in different directions. On 25 May, at 0830, explosions generated ash plumes that rose to an altitude of 9-10 km and drifted 16 km NE. The ACC was raised briefly to Red. Within the next 90 minutes, the ash plume was identified in satellite images drifting 82 km ENE. Strong steam-and-gas emissions rose from the lava dome. The ACC was lowered back to Orange.

KVERT reported that during the last week of May and first half of June, powerful explosions generated ash plumes that rose 8 km in altitude and drifted 550-1,554 km in various directions. Pyroclastic flows traveled 10 km. Ashfall was reported in Klyuchi Village (50 km SW) on 8 June.

On 15 June, at 0425, powerful explosions generated ash plumes that rose as high as 12 km altitude (figure 45). The ACC was raised to Red, and then back down to Orange by the end of the day. Ash plumes drifted 1,000 km NE and SW during 15-16 June. Ash fell in Klyuchi (50 km SW), Maiskoe, Kozyrevsk (115 km SW), and Atlasovo (160 km SW).

Figure 45. Photo of an ash cloud from Sheveluch generated by a powerful explosion that began at 1625 UTC on 14 June 2017. Photo by A.V. Voznikov; courtesy of the Institute of Volcanology and Seismology FEB RAS, KVERT.

According to KVERT, explosions on 17, 18, and 27 June generated ash plumes that rose as high as 7-10 km altitude and drifted as far as 1,500 km. Explosions on 2 July sent ash plumes to 10-11 km; one plume drifted 1,050 km SW and another drifted 350 km NE. On 23 July, strong explosions generated ash plumes that sailed up to 11-12 km and drifted 1,400 km E. Explosive activity the next day lasted about 8 hours and generated ash plumes that rose 11.5-12 km in altitude and drifted almost 700 km NE and 1,400 km E. Strong pyroclastic flows were also noted. The ACC was raised to Red. Later that day, only steam-and-gas emissions with a small amount of ash was observed, and the ACC was lowered to Orange.

Thermal anomalies. Thermal anomalies based on MODIS satellite instruments analyzed using the MODVOLC algorithm were frequent during the current reporting period. From 1 March to 31 August 2016 thermal anomalies were detected 11-20 days each month. The number of days each month with anomalies was lower during 1 September 2016 to 30 July 2017 (except for October with 13 days), ranging from 3 days in April and May 2017 to 10 days in March. Only one hotspot was recorded in July 2017. The MIROVA system detected numerous hotspots every month during August 2016-July 2017, most of which were about 5 km or less from the summit with very low power signatures.

Figure 46. Thermal anomalies at Sheveluch identified on MODIS data by the MIROVA system (log radiative power) for the year ending 4 August 2017. Courtesy of MIROVA.

Geologic Background. The high, isolated massif of Sheveluch volcano (also spelled Shiveluch) rises above the lowlands NNE of the Kliuchevskaya volcano group. The 1,300 km3 andesitic volcano is one of Kamchatka's largest and most active volcanic structures, with at least 60 large eruptions during the Holocene. The summit of roughly 65,000-year-old Stary Shiveluch is truncated by a broad 9-km-wide late-Pleistocene caldera breached to the south. Many lava domes occur on its outer flanks. The Molodoy Shiveluch lava dome complex was constructed during the Holocene within the large open caldera; Holocene lava dome extrusion also took place on the flanks of Stary Shiveluch. Widespread tephra layers from these eruptions have provided valuable time markers for dating volcanic events in Kamchatka. Frequent collapses of dome complexes, most recently in 1964, have produced debris avalanches whose deposits cover much of the floor of the breached caldera.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Far East Division, Russian Academy of Sciences, 9 Piip Blvd., Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/kvert/); Institute of Volcanology and Seismology, Far Eastern Branch, Russian Academy of Sciences (IVS FEB RAS), 9 Piip Blvd., Petropavlovsk-Kamchatsky 683006, Russia (URL: http://www.kscnet.ru/ivs/eng/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/).

Abundant ash emissions, Strombolian activity, pyroclastic flows, lahars, and a few lava flows have all been documented at Tungurahua, which lies in the center of Ecuador. Historical observations are recorded back to 1557, and radiocarbon dates are as old as 7750 BCE. Prior to renewed activity in 1999, the last major eruption had occurred during 1916-1918. Since 1999, there have been numerous eruptive episodes, but only a few with breaks in activity longer than three months. Eight distinct episodes of activity occurred from November 2011 through December 2014 that included 10-km-high ash plumes, Strombolian activity, pyroclastic flows, lahars and a lava flow (BGVN 42:05).

Another eruptive episode during April and May 2015 is described below based on information provided by the Observatorio del Volcán Tungurahua (OVT) of the Instituto Geofísico (IG-EPN) of Ecuador, and aviation alerts from the Washington Volcanic Ash Advisory Center (VAAC). Seismic activity increased after a few months of quiet in late February 2015. A new eruptive episode began on 6 April with tremors and ash emissions, which persisted for the next two weeks. Activity tapered off at the end of April. Intermittent ash emissions and ashfall were observed during May. Rainfall led to numerous lahars every month from January through June, many in drainages on the W flank; some were large enough to disrupt travel on local roads.

Activity during January-March 2015. Tungurahua remained quiet during January 2015, although weather conditions prevented visual monitoring for much of the month. Intermittent gas emissions reached 300 m above the crater a few times. Several hundred millimeters of rain during the second week led to numerous lahars on 9 January that descended the Ulba, Vazcún, Juive, Hacienda, Achupashal, Pingullo, Chontapamba, Romero and Rea quebradas (ravines). Most of the lahars consisted of only muddy water, but three carried debris up to 30 cm in diameter in flows several feet wide, moving material at several cubic meters per second (Palmahurco (Juive), Achupashal, and Rea quebradas). During the last week of January, incandescence was noted at the crater on clear nights.

A lahar on 1 February 2015 in the Yuibug sector, reported by an observer in Bilbao, briefly closed the Penipe-Baños road. Clearer weather at the summit in early February revealed weak gas emissions rising to 500 m above the summit crater. IG reported a gradual increase in seismicity beginning on 16 February 2015. They noted an increase in the number of long-period (LP) earthquakes associated with fluid movement near the summit. They also recorded constant inflation since the beginning of January, with an increase in the rate of inflation of the N flank during February. A small explosion was reported on 18 February, but no other surface changes were observed. The Washington VAAC issued a report of a small burst of possible ash and gas seen by the volcano observatory (OVT) mid-day on 24 February at 5.2 km altitude drifting slowly W.

Observatorio del Volcan Tungurahua (OVT) personnel noted steam and gas emissions during 3-5 March 2015 rising 200-500 m above the crater, but no ash was reported. Rainfall led to a lahar on 23 March that carried 30-cm-diameter blocks down the Quebrada de Juive. Seismic activity fluctuated throughout March. After several months of inflation, a sudden change to deflational deformation began on 26 March, as recorded at the RETU station near the summit crater.

Activity during April-June 2015. Moderate amplitude tremors began during the early morning of 6 April 2015; nearby residents reported noises from the volcano starting around 0730 local time, and minor ashfall was reported in Chacauco, Manzano, and Punzupal Alto. Residents of Palitahua reported a gray ash plume drifting W up to 2 km above the crater (figure 81). The seismic events recorded during the following days were all located at depths of 1-6 km, directly under the crater. Ashfall was reported from 6 to 9 April SW and W of the volcano, primarily in the Choglontus sector. On 8 April, ashfall was reported in Manzano, Choglontus, Bilbao, Chacauco, Pillate, and Quero, with accumulation rates of 135-200 grams per square meter per day (g/m²/day). Ashfall increased in Choglontus, reaching 1000 g/m² during 8 and 9 April. Inflation was again observed at the RETU station beginning on 5 April.

Figure 81. Ash-bearing emissions from Tungurahua drift NW on 6 April 2015. Photo by J. Garcia, courtesy of OVT/IG-EPN (Informe No. 789, Síntesis Semanal del Estado del Volcán Tungurahua, del 31 de marzo al 07 de abril del 2015).

The webcam revealed continuous emissions of ash beginning on 6 April 2015. A plume was reported at 6.1 km altitude moving W until the following day. On 8 April OVT reported dense ash emissions to 5.9 km altitude, drifting NW. Weather clouds prevented observation in satellite imagery during these days. Local aircraft indicated ash present at 6.7 km altitude on 9 April in spite of extensive weather clouds. A swarm of "drumbeat" LP earthquakes on 10-11 April was followed by moderate ash emissions on 12 April. On 11 April, IG reported an ash cloud moving W and NW from the summit at 5.5 km altitude. An emission on 14 April with moderate amounts of ash rose 500 m above the summit and drifted WSW (figure 82).

Figure 82. An ash emission rises to 500 m above the summit at Tungurahua on 14 April 2015, and drifts WSW. Photo by B. Bernard, Courtesy of OVT/IG-EPN (Informe No. 791, Síntesis Semanal del Estado del Volcán Tungurahua, del 13 al 21 de abril del 2015).

An explosion on 15 April generated an ash plume that reached 3 km above the summit crater (figure 83). Later in the day ash emissions rose to 2 km above the crater and drifted W. The Washington VAAC reported a continuous ash plume visible in satellite imagery on 15 April moving W from the summit at 6.1 km altitude (1 km above the summit). Although the plume appeared to be almost 100 km long, ashfall reports were limited to areas within 15 km of the summit. Collected ash was mainly composed of dense lithic fragments, euhedral crystals, and oxidized particles, and was not considered juvenile material (from fresh magma). Additional ashfall was reported up through 17 April in Palictahua, El Guanto, El Mirador, El Santuario, Mapayacu, Puela, Chontapamba, and Sabañag.

Figure 83. An ash plume from an explosion at Tungurahua rises 2 km above the summit crater on 15 April 2015. Photo by B. Bernard, Courtesy of OVT/IG-EPN (Informe No. 791, Síntesis Semanal del Estado del Volcán Tungurahua, del 13 al 21 de abril del 2015).

Ash emissions continued at a lower level of frequency and energy after 17 April 2015 (figure 84), and seismic activity notably decreased. There were minor emissions coincident with seismic tremors that produced gray to black fine-grained ashfall mainly to the W of the volcano in Bilbao and Chontapamba through 27 April. Deformation changed from inflation to deflation beginning on 21 April, but after five days, switched back to inflation on 26 April. Plumes with moderate ash content were observed rising to 1 or 2 km above the summit on clear days. An emission on 28 April contained modest amounts of ash and drifted NW.

Figure 84. A double column of steam and red-brown ash rises 500 m above the crater at Tungurahua and drifts W on 17 April 2015. Photo by B. Bernard, Courtesy of OVT/IG-EPN (Informe No. 791, Síntesis Semanal del Estado del Volcán Tungurahua, del 13 al 21 de abril del 2015).

Intense rains occurred on 25 and 26 April 2015 that were large enough to generate significant lahars. On 25 April, lahars were reported in the Chontapamba and Romero ravines moving blocks up to 1 m in diameter. Muddy water was observed in the Achupashal ravine. On 26 April, lahars were reported in the Juive, Mapayacu, Romero, and Chontapamba drainages. Lahars caused a high-frequency seismic signal from the Pondoa ravine during the late morning. The flow rates doubled in Vazcún and Puela ravines, which filled with muddy water. A large lahar was also reported in the Quebrada del Pingullo, and debris was reported in the Clementina, Juive Chico, and La Pampa creeks (figure 85).

Figure 85. Mud and debris filled La Pampa quebrada (ravine) at Tungurahua after heavy rains on 26 April 2015. Photo by S. Aguaiza, courtesy of OVT/IG-EPN (Informe No. 792, Síntesis Semanal del Estado del Volcán Tungurahua, del 21 al 28 de abril del 2015).

During the first week of May 2015, constant steam emissions rose 1 km above the summit crater. The vapor was characterized by very low amounts of ash. On 4 May, ashfall was reported in the Bilbao sector, but not corroborated from other areas. Steam with low to moderate ash content continued through 12 May, with plumes rising 1 km above the summit, mostly drifting W and SW. As a result, ash falls were reported in Manzano, Choglontús, Yuibug and Bilbao. On 10 and 11 May, intense and prolonged rains led to significant lahars in Q. Romero, Ingapirca, Chontapamba, Achupashal, and Ulba, and smaller lahars in several other ravines. Small mudflows and lahars also occurred in the ravines on the W flank on 12, 14, and 15 May. Cloudy weather mostly prevented views of the summit, but continuous steam emissions were observed when it cleared. Fine-grained gray ashfall was reported in Choglontus on 15 and 20 May.

Minor emissions of steam with no ash to 500 m above the crater, drifting mostly W, persisted throughout June. Intermittent rains resulted in minor lahars and mudflows that caused little damage. Lahars descended ravines on the W flank on 16 and 17 June. The summit was cloudy and rainy for much of the month, and seismic activity remained low.

Geologic Background. Tungurahua, a steep-sided andesitic-dacitic stratovolcano that towers more than 3 km above its northern base, is one of Ecuador's most active volcanoes. Three major edifices have been sequentially constructed since the mid-Pleistocene over a basement of metamorphic rocks. Tungurahua II was built within the past 14,000 years following the collapse of the initial edifice. Tungurahua II collapsed about 3,000 years ago and produced a large debris-avalanche deposit to the west. The modern glacier-capped stratovolcano (Tungurahua III) was constructed within the landslide scarp. Historical eruptions have all originated from the summit crater, accompanied by strong explosions and sometimes by pyroclastic flows and lava flows that reached populated areas at the volcano's base. Prior to a long-term eruption beginning in 1999 that caused the temporary evacuation of the city of Baños at the foot of the volcano, the last major eruption had occurred from 1916 to 1918, although minor activity continued until 1925.

Information Contacts: Instituto Geofísico (IG), Escuela Politécnica Nacional, Casilla 17-01-2759, Quito, Ecuador (URL: http://www.igepn.edu.ec/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/, archive at: http://www.ssd.noaa.gov/VAAC/archive.html).